as far as mastodon instances the biggest one is mastodon.social but it is often filled with spam. atm I have accounts in kolektiva.social (mainly anarchism/political stuff) and functional.cafe (functional programming basically, tho a bit off-topic sometimes)

I also havean blahaj.zone account so I use calckey (hajkey) too and you can interact with mastodon instances because of the activity pub (in a way, the glue that makes all of the federated platforms stick). calckey has a lot of features that mastodon doesn't have so you may find it difficult in the beginning, that being said, its really powerful and has the capacity to be a lot of fun