Is there some kind of tutorial or explanation for beginners who only came here because of sync and don't understand anything that's going on.
On one side this experience is really simple and easy, sync makes it very much like Reddit. But I see some people have @signs in their names showing their on different instances... Does that mean my comments aren't seen by all people and what the heck is an instance and what an I doing?
I've seem this metaphor around and will try to replicate here. Lemmy (and kbin) instances are like email providers, you can have an email from Google, outlook, Proton, etc. and they all can send emails to each other. Lemmy instances are like this, if you have an account in an instance, you can see content and interact with all the other instances.
Some instances don't talk to each other (defederate) for a myriad of reasons, but mostly because of political opinions and trolls. Here's a link with a more in depth explanation of how defederation works, from when it happened with one of the big instances on lemmy: https://lemmy.world/post/149743
I think only the admins of the instance and mods of the community (and you) can delete your comment. Afaik your comment is only cached on other instances.
Anyone can create a community. From the looks of it, sync doesn't support it yet(?). But if you go to your instance on your browser you can create it from there on the top right settings.