idk where to find a crashcourse but here are my opinions(!) on top of my head after nearly 3 years on different instances:
- hexbear and lemmygrad are to be avoided by all costs. weird tankies. they are blocked by most instances either way.
- .ml(stands for marxist leninist) is the instance of the lemmy devs(i think?) and is still populated by a lot of non-lunatics. But i blocked the whole instance either way.
- idk what the deal is with quokk.au(?) users but i noticed over the years that they tend to post weird shit/takes.
- kbin was a fediverse alternative to lemmy. kinda fell apart a few years backs. a few instances are still around. they are alright.
- mbin is the successor/a fork of kbin. if you want to try a different ux to lemmy its worth a look.
- beehaw was already a pretty tightly knit community before the reddit api debacle a few years back. they have self proclaimed higher posting standards.
- db0 is an anarchist instance. good for piracy topics. otherwise i dont care or have an opinion on them. tend to be a bit more ai tolerant i believe.
- lemmy.world was/is the biggest instance. they get/got a lot flack for some reasons and others. most of the drama was imho pretty overblown.
- feddit.org is an european instance. most of its users are german.
- piefed is yet another fediverse alternative. i like it.
otherwise there is stuff like lemdroid with a focus on android or instances with a focus on anime. but tbh just start using lemmy and make your own experiences and feel free to block all the weird shit you dont want to see.