Best I can say is that you have your local server, which is the server you originally signed up in. You then have all the rest of the connected (federated) servers that can see what you say.

Each server is its own mini-lemmy, so it is broken down into communities just like subreddits. Each has its own server administrators and each community has its own moderators.

Regarding content delivery, it seems to be about the same process: a link, image, video, or text from a certain community. The responses are similarly threaded with buttons for interaction.

The largest differences I have seen are that each server instance can have duplicate communities. Different servers have different tolerances for communities. In extreme cases, whole servers can be blocked (defederated) from your server. This is usually due to unwanted behavior from the users that is not curbed by the moderators or extreme communities that cause hardship for other users.

This is what I got after about a week of lurking. If I'm not correct, please let me know.