The other comments explain it well, but if you’re still confused, the easiest way to understand it imo is to just think about it like emails. Gmail, Yahoo, Outlook, Proton, these are all completely different websites, but they serve the same purpose and can interact with each other regardless. You don’t need a Gmail account in order to send your friend on Gmail an email. On the other hand, you can’t sign into Gmail with your Yahoo account.

Which one you sign up for is usually down to personal preference regarding their rules and website design, though this isn’t always the only reason to pick. If Google one day comes out as being run by Nazis, some email providers might choose to “defederate” Gmail, essentially blocking the ability to receive emails from them. If you don’t wish to receive emails from Nazis, it’s probably a good idea to pick one of them over one that doesn’t block Gmail.

Just imagine that but for a website like Reddit and you’ve got Lemmy.