Well I'd say most of them are federated together, or at least those with a good amount of users. In practice you don't really get islands other than I guess troll instances that everyone has blocked.
And AFAIK as long as an instance isn't blocked by yours (and vice versa to be useful), you can follow a person on that unfederated instance and it should just work and get federated.
Wait is something like that actually in Project 2025?