1. No, you don’t need. It is the concept of federation
2. Yes it does

It is explained on community instances

That mean, you have to go on lemmy.ml and search [email protected]

3. Look for communities in all instances, beehaw.org is nice. If you speak languages other than english, there might be little communities in your languages. I know feddit.de is pretty big if you speak german for example

4. It depends of your browser. For firefox and for chrome