most people have answered your questions so i want to chime in with the information that i wish someone had told me when i first joined:

a lot of people came to lemmy from reddit like you and i both did and also mostly for the same reasons. most of them went to lemmy.world because it was the first search result on the big search engines like google & bing. those people have turned lemmy.world into a mini reddit and ended up recreating the same problems that reddit has plus more; hence the bot check that you ran into when you signed up.

the original instances of lemmy all have a strong leftist bent; i think of it like if r/politics; r/anarchy/; r/communism; r/socialism; etc. went off and created another social media platform and then started discussing everything like reddit does, but from this perspective. instances is the name given to individual servers and all those servers combined is nicknamed the lemmyverse, or lemmy, for short.

the fediverse is the nickname given to the pubg protocol that's shared between all the platforms that use it like lemmy, mastadon, kbin, threads, bluesky, etc and that means that the conversations from all of those platforms are shared amongst each other so it's possible to be on lemmy and have a conversation with someone on kbin, for example. i stick with lemmy because it's doesn't have any venture capital investors pushing the admins to enshitify it to maximize profits like has been happening to reddit and bluesky; i've been moving from one social media platform to another because of enshitification like reddit's since the 1990s (before it was called social media) so this last part matters to me a lot.

i started off on lemmy.world like most ex-redditors did and discovered that they've duplicated the censorship thing that reddit likes to do with defederations so i switched to lemmy.ml since it doesn't defederate with anybody due to fact they're the primary instance where lemmy development takes place. the federation is what makes lemmy decentralized and when you defederate; you cut yourself off from the rest of the lemmyverse, but lemmy.world and some of the other instances that got most of the ex-redditors like the star trek instance use it to try cut off content and people from the instances that they don't like and that's their right since it's their instance. lemmy is decentralized so trying to cut out people & content only serves to cut yourself off and that's intention behind the fediverse; to make it so that no power tripping mod or ban happy admin can stop the conversation like they do on reddit.

everything is done by volunteers and donations and, if you don't like one instance; you can move onto any other one and still get a similar experience. i don't like letting other people decide what i can & can't see and who i can & can't talk to so i mostly stick to the instances that don't defederate with anybody like lemmy.ml and i use the block-people and block-communities features when i feel like i need them for myself.