Well the best way to think about this is not "occupation" but "colonization" because Japan did colonize Korea much as the French colonized Algeria and the British colonized India (and a bunch of other places).

Colonialism is, by nature, exploitative and violent and Japanese colonial presence in Korea was no exception. Japan did modernize Korea in terms of infrastructure, education, urbanization, etc. and later industrialization, but it was always for the benefit of Japan not Koreans. So, for example, Japanese colonial officials in the 1910s launched the first modern land survey in order to create a full-scale modern tax system that any modern centralized government needed to function. This meant, however, that many illiterate Korean farmers lost their lands and became impoverished tenant farmers because they couldn't provide the "modern" land deeds to "prove" their ownership. On the other hand, Korean landlords actually got richer because they teamed up with Japanese land speculators in seizing land from poor farmers.

In the 1920s, Japan founds the first modern university in Korea - Keijo Imperial University. But admissions was hugely biased in favor of children of Japanese settlers.

Overall, Japanese colonial rule in Korea was harsh - but then when we look at it from a global perspective, it was harsh because colonialism is violent and not because the Japanese were especially evil.

You asked for the "real horrors" of Japanese colonialism in Korea but, except for comfort women and slave laborers, the realities of colonialism were much more mundane. What I mean is, it's not like the Japanese were going around killing Koreans en masse. It was the little, petty indignities Koreans had to endure every...single...day. Discrimination in employment, education, welfare benefits. Intimidation by Japanese police. Random health inspections by sanitation experts who can barge into your homes in the name of public health.

But also remember that colonialism is complex and in the Korea case, colonialism evolved over time. The first nine years were known as the period of Military Rule (1910-1919), characterized by harsh military governance. The next decade and a half was the so-called Culture Rule (1919-1937 thereabouts), in which the colonial government adopted a more conciliatory tone and tolerated a limited revival of Korean-language newspapers, magazines, and books. The 1920s and 1930s saw a renaissance in a way of Korean civil society but fractured through the distorted prism of Japanese colonial domination - what scholars call "colonial modernity."

The third and final phase of colonialism was war mobilization (1937-1945), when the colonies were mobilized by Japan for total war. Korean bodies mobilized as sex workers and laborers for the Japanese war machine. And this coincided with a renewed push for assimilation - (kominka undo) - in which Koreans were forced in various ways to shed their Korean identities and adopt Japanese ones.

But even here there was a complicated and usually ignored wrinkle. Some Koreans were able to take advantage of Japanese colonialism and create a better life for themselves. Many Koreans followed the Japanese armies and settled into Manchuria, Beijing, and Shanghai to take up new, middle-class professions like doctors, lawyers, or entrepreneurs (starting what would later become South Korea's middle class). A few Korean capitalists joined forces with the Japanese colonial government and became huge industrial conglomerates because of World War II, and the direct ancestors to some of the chaebol today (Ko'chang Kims are the most famous example).

This is not to justify Japanese colonialism. It is to say that when we focus solely on the "horrors" of Japanese colonialism in Koreans, in a way it kind of demeans and infantilizes Koreans themselves. It creates a story of Koreans as always being helpless, passive victims to history when we know that was not the case. Not a few Koreans refused to quietly accept their unfortunate fate as colonial subjects and learned to "work the system" in order to create some kind of better life for themselves. Again, this is not a justification for colonialism but a reminder that real life is, frankly, complicated.

On your last question about Koreans in Japan during colonial times (at least, I think that's what you're asking), interestingly Koreans who arrived as factory workers or students strangely enough enjoyed more political rights than Koreans in Korea. They still faced discrimination of course but at the universities, Korean students were exposed to Marxism and socialism and became quite active anti-colonial nationalists later on. And there was even one Korean who was elected to the Japanese Imperial Diet in the 1930s.

By the way, the case of Taiwan is a little different from Korea. Another reminder that even within a single empire, colonies were administered differently.