Were Confederate soldiers plantation or slave owners?

Not all of them were, but many of them were. As I'll get to, men from slaveholding families were more likely to serve than men from non-slaveholding families, and men from plantation families were slightly more likely to serve than men from slaveholding families of more modest means.

In absolute numbers, though, plantation owners participating as active duty soldiers were relatively rare, because plantation owners themselves were rare in the first place. They also tended to be older men, beyond fighting age in many cases, and even if not, then very often beyond conscription age. Even so, plantation owners were certainly present among the Confederate forces. Robert E. Lee is the most obvious example, but many other Confederate generals, as well as other senior military officers, and certainly those holding Confederate political office, were plantation owners. Further, many sons of plantation owners served, even if they themselves didn't technically own people as slaves.

In his book General Lee's Army: From Victory to Collapse, historian Joseph Glatthaar includes a study of the rate of slave ownership among Lee's Army of Northern Virginia. I'll quote the relevant paragraphs:

"Even more revealing was their attachment to slavery. Among the enlistees in 1861, slightly more than one in ten owned slaves personally. This compared favorably to the Confederacy as a whole, in which one in every twenty white persons owned slaves. Yet more than one in every four volunteers that first year lived with parents who were slaveholders. Combining those soldiers who owned slaves with those soldiers who lived with slaveholding family members, the proportion rose to 36 percent. That contrasted starkly with the 24.9 percent, or one in every four households, that owned slaves in the South, based on the 1860 census. Thus, volunteers in 1861 were 42 percent more likely to own slaves themselves or to live with family members who owned slaves than the general population.

"The attachment to slavery, though, was even more powerful. One in every ten volunteers in 1861 did not own slaves themselves but lived in households headed by non family members who did. This figure, combined with the 36 percent who owned or whose family members owned slaves, indicated that almost one of every two 1861 recruits lived with slaveholders. Nor did the direct exposure stop there. Untold numbers of enlistees rented land from, sold crops to, or worked for slaveholders. In the final tabulation, the vast majority of the volunteers of 1861 had a direct connection to slavery. For slaveholder and nonslaveholder alike, slavery lay at the heart of the Confederate nation. The fact that their paper notes frequently depicted scenes of slaves demonstrated the institution's central role and symbolic value to the Confederacy."

He goes on to write that "more than half the officers in 1861 owned slaves" as well. This percentage, though, did go down after 1861, as the Confederacy enacted a draft, and as the Confederacy needed to force those less willing to fight into service, in order to have enough troops to remain effective.

Similar findings are presented in the paper "Wealth, Slave Ownership, and Fighting for the Confederacy: An Empirical Study of the American Civil War" by Andrew B. Hall, et. al. While the paper isn't concerned with the overall percentage of Confederate soldiers who held people in slavery, it examines the question whether a Southerner who was a slaveholder was more likely to serve the Confederacy than their non-slaveholding Southern counterparts. The study finds that, yes, young men from slaveholding families were more likely to serve: "the propensity to fight in the Confederate Army is lowest for those households owning no slaves." Interestingly, the study also finds that the number of slaves a family held didn't substantially increase the likelihood of serving. That is, the probability jumps significantly between a man coming from a family owning zero slaves, and a man coming from a family owning at least one slave. But the difference is rather modest between families who owned between 1-3 slaves, and families who owned between 4 and 1,000 or more (though the probability still does rise a bit).

But after 1861, this probability was affected by Confederate legislation. In late 1862, the Conscription Act in the South included a provision known as the "Twenty Negro Rule". This was in reaction to Abraham Lincoln's Preliminary Emancipation Proclamation - the South was always wildly afraid of "servile insurrections" and they believed Lincoln's declaration would have the effect of a large-scale slave revolt. To combat this threat, the Confederacy exempted one white male from the draft for every twenty enslaved people held per family. That is, for families who held twenty enslaved people, then the slaveholder or one of his sons could be exempt from the draft. For families who held a hundred enslaved people, then five total draft-eligible men in the slaveholder's family could be exempt.

As Eric Foner explains in his book Reconstruction: America's Unfinished Revolution, 1863-1877 (as does Glatthaar in his book), this was a controversial provision among Southerners, who often expressed animosity toward the plantation class over this—they were fighting primarily for the plantation class's benefit, while that plantation class was exempt from fighting. Thus, the provision was repealed, but not until after it had the effect of exempting many in this class of slaveholders at a critical stage in the war. Nevertheless, as Hall's study finds, this plantation class was still more likely to serve the Confederacy than the average young men from non-slaveholding families as well as the young men from slaveholding families who weren't from the plantation class, even if the difference with the latter was rather modest.

If not, which I'm assuming they're not because the slave owners were the upper Rich class and they didn't fight in wars, why were the Confederate soldiers going to war? What did they have to benefit making sure a bunch of rich people were allowed to keep slaves

This is a question I have answered in this sub before. Feel free to read that whole post to get a more thorough explanation and sources. Suffice it to say, "because my family owns people as slaves" was only one of the many appeals and justifications made on behalf of the preservation of slavery as the Southern cause. To summarize other appeals and justifications made on behalf of slavery that I explained in that earlier post:

• "If slavery ends, then you'll be competing with jobs against former slaves, saturating the job market and driving wages down. They're gonna take your jobs and leave you even more destitute than you already are! You don't want that to happen, do you?"

• "If black people are free, then they'll be equal to you, but you are superior to them. You don't want to be degraded to the same level as a black person, do you?"

• "If black people are free, they'll get the vote, and they'll elect black politicians, who will take their revenge on white people. We'll have no political control, subjected to the laws and desires of the North and the black South. Do you want to be slaves to the North and their Southern black allies?"

• "The North wants your daughters to be able to marry black men, and then your whole family will be black, and no better than a slave. You don't want to see a mixed race Southern society, do you?"

• "The end of slavery will mark the beginning of a race war that will either wipe out the white South or the black South, and the South could end up like what happened in the Haitian Revolution. You don't want to see a genocide of the white South, do you?"

• "If you work hard, you can be a slaveholder, too, one day, and the North wants to take that opportunity away from you. You want to preserve your prospects of upward mobility, don't you?"

There were other pro-slavery appeals made as well, but those were probably the most common. There were other non-slavery appeals made by the Confederate leadership, too, though the other most common cause—"liberty"—was tied up with slavery as well, as explained in more detail in that previous post. "Liberty" to Southerners often meant "the liberty to own people as slaves". And "liberty" meant "liberty from the emerging political dominance of the North over us, who intend to take our slaves away". Which was why secession and then the war occurred in direct reaction to the election of Abraham Lincoln to the Presidency and his band of "Black Republicans" who had a majority in Congress. Southern "liberty" meant liberty from being ruled by a (possibly permanent) anti-slavery majority at the federal level.

slaves that they weren't even benefiting from?

Non-slaveholding Southerners benefited from slavery in a variety of ways—employed by slaveholders, providing services to slaveholders or plantations (from blacksmiths to doctors to anything in between), renting slave labor by the day/week/month, etc. This was another reason why the above pro-slavery appeals worked on the non-slaveholding class.

There's always more to say, but I answered a question a few days ago, which addresses some of these questions.

The very short answer is that many Confederate soldiers came from slave owning households, even if they didn't personally own slaves themselves (as technically only the individual who directly owned a slave - usually a male head of household - was the "slave owner").

Slave ownership was not restricted to the very wealthy, and while enormous plantations enslaved hundreds of people, the majority of slave owners were of more modest means, owning 5 or fewer slaves. Those soldiers who didn't come from slave owning households often benefited from the practice of slavery indirectly, as it was common to work for slave owners, rent land or slaves from slave owners, or otherwise benefit from the practice of slavery without personally owning any slaves.

Further, a 2018 study of Confederate data found that slave owning households fielded more soldiers in the Confederate army than households that did not own slaves:

"We assemble individual-level data on roughly 3.9 million free citizens in the Confederate states alive prior to the outbreak of the Civil War. Our data records information about each citizen’s wealth, the number of slaves owned, occupation, family relationships, and, for men, an estimate of whether or not each fought in the Confederate Army. Using this data, we show that households that did not own slaves fielded fewer Confederate Army soldiers, on average, than did households with slaves. Households reporting no real-estate wealth in the 1850 census likewise fielded fewer Confederate soldiers, on average." [page 3]

[...]

"Our results contribute to a longstanding historical debate about who fought for the Confederacy. The old saying that the Civil War was a “rich man’s war but a poor man’s fight” may hold water, perhaps when applied to the very wealthy, but our evidence suggests that modest increases in wealth and in slave ownership actually made southern white men more likely to fight, not less. A majority of the Confederacy did not own slaves, but those who owned slaves fought at higher rates. The familiar observation that many soldiers were non-slaveowners largely reflects that most southerners were not slaveowners, but this does not imply that non-slaveowners supported the Confederacy more." [page 31]

In addition to these excellent answers, Stephanie McCurry's book, "Masters of Small Worlds," dives deep into why yeoman farmers who had at most a handful of enslaved people working for them bought into secession and the CSA overall. It was a matter of the constructions of manhood and their desire to associate/affiliate themselves with the planter class, to move up in the world, and that meant fighting for the slave society that kept them up above the very bottom, even if they were incredibly poor. I believe she also addresses this in "Confederate Reckoning," but I'm not 100% certain. She's a great resource for understanding the social and economic choices non-elite whites made regarding slavery and the Confederacy.