You speak of "primary" and "secondary" as if it were an absolute property of the source itself, which it isn't. It depends entirely on what you're talking about.

You should look at what the sources actually are (and aren't) here. First off there are two "Eddas" and their nature as sources is very different. The Elder or Poetic Edda was compiled probably in the late 12th century and consists of poetry that was passed down in an oral tradition. It's generally accepted that a lot of this was upwards of two centuries older. The language is archaic and that makes sense, as poetry tends to be pretty conserved, as people remember it by heart and can't just substitute words without screwing up the meter. So on linguistics grounds these texts can and have been dated, although not consistently. The whole historiography of their dating is a story in its own right but in short, they used to be thought much older than we believe they are today, where it's perhaps 10th century at the oldest. At least one text (Rígsþula) is also generally considered to have been written in the 13th century despite archaic language.

The Younger Edda, Prose Edda or Snorri's Edda is not a text that was passed down. It was composed by Snorri Sturluson in the early 13th century. There are a number of reasons why Snorri wrote it (discussed in the thread Why did Snorri write the Prose Edda? although inevitably I'm going to repeat a lot of what I said there) but the main one was that he wanted to preserve the tradition of writing Skaldic poetry, of stories about the gods-as-folklore-characters. It was not Snorri's intent to document the pre-Christian religion, which is a different matter. These stories were about the gods but they were never religious scripture and they were told for entertainment purposes, not ritual ones. It is only through occasional and often vague references in the texts.

I don't think it can be emphasized enough that Snorri did not believe the old gods to be gods. He engaged in euhemerism, believing the old gods to have been human kings, some with magic powers, who'd achieved great reknown and whose reputation in life and after it had become greatly exaggerated. That wasn't a unique perspective to him but how former-Roman parts of Europe had been relating to their own pagan pasts, and pre-Chrstian literature like Virgil's Aeneid enjoyed enduring popularity at the time. (and is referenced by Snorri too) Snorri wasn't even first in Scandinavia to euhemerize the gods. Saxo Grammaticus in his 12th century history of the Danes had given his historical theory on who the gods had been, partially rooted in different versions of the same folklore. So the story of Baldr for instance is very different in Saxo's account.

Which is another important point, which was that no single coherent Norse Mythology ever existed. Or even a single Norse culture for that matter.

That said, as I wrote in the other thread, we also know that to at least some extent, Snorri did have access to some sources otherwise lost to us. Only Snorri's account of Thor's fishing trip mentions Thor's foot going through the bottom of the boat, but that detail is depicted on the Altuna Runestone (U1161) from the 11th century.

That does not mean one should blindly trust Snorri as having deep knowledge. Historically there's been a lot of doing just that to an extent that can appear naive. For instance, reading in aspects of Old Norse beliefs in Ynglinga Saga without taking into account that it's a story composed by Snorri where Odin and company in are not the gods but the 'actual' humans the gods were based on.

Another example is for instance the Æsir-Vanir distinction as two groups of gods, and in more recent times Rudoph Simek (whose an authority) has argued (fairly convincingly IMO) that this whole thing, which you'll find stated as fact in just about any popular book on Norse mythology, is in fact Snorri's own invention and that vanir was just a less-common/poetic term for gods that he'd interpreted as a separate group, and concocted a historic theory (in Ynglinga Saga again) to explain their origins.

Snorri is to some extent a primary source on what the folklore was in his day. Some of which does go back to the Viking Age, and some of which was composed in the intervening two centuries. But he was not a folklorist who was just recording what he'd heard, there can be no doubt he was trying to synthesize it into a coherent whole, and a historic theory to explain it. But he did not draw on oral accounts alone but very heavily on the Poetic Edda. Snorri was not clearly not source-critical either; in several points Ynglinga Saga appears to go out of it way to give 'historic' justifications for the insults hurled by Loki in Lokasenna in the Poetic Edda. (e.g. Odin's wife cheated on him but they all thought he was dead, Freyja engaged in incest but that had been an accepted custom among the Vanir)

Or, in another place Snorri states that Freyja takes half of the dead and half go with Odin to Valhalla. No story or mythological justification is given here, and it comes across more as an attempt to reconcile different stories/traditions.

So in summary, the Poetic Edda is a bunch of poems out of the ones preserved in Iceland by 1200. Snorri's Edda is a 13th century book on how to compose traditional poetry, where some of the mythology known at that time is given as background information to draw on. They're not considered sources on any historical events, to the extent they even purport to describe things that actually happened.. The Fornaldarsögur, sagas of the 13th and 14th centuries that take place in the Viking Age are in modern Scandinavian historiography considered almost completely useless as sources for that period. (and this has been the case for most of the last century since the school of criticism founded by Lauritz Weibull won ground). None of the kings in Ynglingatal are considered to have existed now, and most would consider it a fool's errand to try to discern the 'real' figures behind these stories.

Another issue is class. The skaldic poetry was written for entertainment at the courts of the well-to do. The poems and sagas were written by, for and about the wealthy elite. It is their values and world view that is reflect. If the sagas have apparently little care for whether someone as a Dane, Swede, Geat or Norwegian (or the even smaller local cultures those were divided into), it's because that class did. They were cosmopolitan and interacted and intermarried both in the Viking Age and ensuing Middle Ages. It is among that elite, the high status burials, that you have a relatively homogenous customs across Scandinavia, with great burial mound and ship burials. Surveys of the archaeological evidence of burial customs have shown that ordinary people during the Viking Age had a diverse set of highly local burial customs, which hint at heterogenity when it comes to culture and religion. Surveys of theophoric place names show great variations in which gods were popular where, and for instance that Ullr was a hugely venerated god, a status that's incongruous with the scant mention of him in the Eddas. (Meanwhile there's no evidence whatsoever for Loki being the subject of any cult or having existed as a god in any practical respect) Some, like the archaeology professor Anders Andrén would even go so far as to consider the whole Norse pantheon as an upper-class concern that had only a tenuous relation to the religion that actually practiced by most people. We don't know that vikings thought they'd go to Valhalla if they died in battle. We don't really know whether this was a core pre-Christian belief, or whether this was just a syncretism, a "viking-ized" version of heaven constructed under influence of Christianity a the end of the Viking Age.

The fact that the Eddas were written down by Christians is fairly irrelevant; First since, as said, Christian influence did not begin there. But secondly and more importantly because the Eddas are not scripture. They are not (and don't purport to be) religious texts or manuals . They occasionally reference pagan religious practices, but never describe them in any detail. We know from archaeology of a number of phenomena and objects that had great apparent ritual importance of which no mentions are made in the Eddas whatsoever. Such as amulet rings, which've been found by the hundreds, deposited over centuries at the larger cult sites excavated. Remember we're not talking about Protestantism here; there's no sola scriptura. It is ritual that is at the heart of the pre-Christian religion here, not scripture or stories or even faith. (to put it another way: Odin wants his sacrifices but doesn't necessarily care if you believe in him)

The Eddas are primary sources on Norse folklore in Iceland circa 1200 CE. As said, some of that goes back to the Viking Age. Some was composed at the time they were compiled, and some in-between. They are not primary sources on the pre-Christian religion, because as said, the folklore's not the religion. That's why they were recorded! As Christians they probably saw little or no value in recording or preserving knowledge of actual paganism. What Snorri obviously did see a value in, was preserving the literature that'd been passed down. Just as the ex-Romans down in Paris had done with Virgil and Ovid. (Paris very much being the intellectual and religious capital for northern Europe at the time)