In my experience, they're split between the Mastodon side of the Fediverse and the Lemmy side. On Kbin, which bridges both, and where you can follow users, you end up with a pretty rich feed if you follow enough people and places - I've got about 100 communities subbed and 300+ users and my feed is better than Reddit's ever was.
Another thing to note is that your instance benefits from your discovery - it doesn't start indexing posts from another instance until someone from the first instance subscribes to it. This means a lot of smaller instances get lost in the shuffle, but they're out there - you just have to find them. I've noted that this is much more difficult to do on my Lemmy account than my accounts with other software, so you might benefit from an instance jump if things feel dry on your current account.