Can someone ELI5 how lemmy instances work?

Some of the things I'm concerned about are browsing across unconnected instances - will I need twenty accounts to follow all of the groups? What is the likelihoood of an instance dissapearing? How do you gauge the culture of an instance? Is the https://redditmigration.com/ actually being populated by real admins of those subreddits? Are there any gotchas from joining an instance that I should be aware of? Thanks!