IIRC there were a couple of nazi instances that were instantly defederated by everyone except each other. But I'm pretty sure they are both dead now. Good.

You could host your own and decide for yourself with which you want to federate. Or just block whatever you don't like.

Easiest is probably to just subscribe to the communities you like and only browse the subscribed communities. And then block instances/users you don't want to see.

Only other thing I have seen is instances not federating at all. Lemmy and probably Piefed as well support that out of the box. In that case they are just normal forums.

Don't know any Lemmy server that does that but Truth Social is actually running Mastodon and not federating with the rest of the world. Not that anyone would want to federate with that shit show anyways.

There arent many that fully isolate itself and defederate with all other servers. Beehaw was of the earlier ones that approached the federation with strict moderation walls.

Check out this graph made by /u/Zedstrian@sopuli.xyz. It shows defederations between major Threadiverse instances. I don't think any instance groups are completely isolated, but some like Lemmygrad and Hexbear have been defederated by over half of the major instances.

How do you mean "the ones we see here"?
You can also poke around fedidb: https://fedidb.com/servers?limit=20