Given how toots are distributed, if you most want to interact with a specific community, it seems like a good idea to join an instance focused on that community.

Toots from people you follow appear in your Home timeline.

Toots from others on your instance appear in your Local timeline.

Toots from people followed by others on your instance appear in your Federated timeline.

The Local and Federated timelines are very instance-focused.

Source: https://medium.com/@TheAnalyticalLeft/mastodon-basics-by-someone-who-didnt-create-it-3d701145df12

Mastodon Basics (by someone who didn’t create it) - Tracy Hall - Medium

Mastodon is both different from AND extremely similar to “the bird app” Skip the blather, just get me started: see tl;dr; below. Mastodon is built *exactly* like the bird app, but on Mastodon you can…

Medium
On the other hand, joining a general instance seems like a good way to get a glimpse outside your personal bubble.
@dhemery I thought federated was what ever your instance syncs with through activity pub not required to have anyone following?

@blackfire You may be right. The mastodon docs say, "When you browse the federated timeline, you see all public posts that the server you are on knows about. There are various ways your server may discover posts, but the bulk of them will be from people that other users on your server follow." —https://docs.joinmastodon.org/user/network/#timelines

So it looks as if "posts from people that other users on your server follow" is the primary way, but perhaps not the only way posts appear in the federated timeline.