Koalas eat mainly eucalyptus leaves. They do have a reputation for being picky eaters. I believe out of the hundreds of different Eucalyptus species they prefer to eat only a few dozen species. But they can also be found on other tree species occasionally.

I've seen Reddit posts that mention this idea that a Koala is too stupid to recognise leaves that are on the ground. I haven't found any reputable sources for the claim though.