That's not the default behavior. Mastodon has 3 built-in timelines:
- Your "home" TL with people you follow, and the stuff they post/repost/etc.
- The "local" TL with the users on your server.
- The "federated" TL that's the union of all the people that users on your server follow. Like if you and I are the only 2 people on that instance, and you follow {A, B, C}, and I follow {B, C, D}, then the federated TL will have posts from {A, B, C, D}.
That's it. There's no proactive reaching out and pulling in content from the rest of the fediverse. The one other possibility would be if you configured your instance to use a "relay" of content from other instances you wouldn't normally connect to, which is something optional you have to add.