I think you're getting hung up on the word "federations" (noun) instead of the adjective "federated".
Who decides who gets to email who? The email provider admins. Should everyone be in a single email network/bubble since otherwise there is no communication? Mostly, yes. Do I need a separate account per email bubble? Per email bubble? Yes. But how many email bubbles are there? One? Whats the practical limit on number of providers per the email world? None, mostly?
Gmail does ban a lot of small email providers if they don't seem "legit" enough. And that is where you're onto something with the noun federations.
If a bunch of instances really disliked a different bunch of instances they can indeed severe each other from each other. The admins would do that. They put the other instances on a block list. Most Mastodon instances block Trump's Lie ehm Truth Social etc. But otherwise you can talk from gmail to hotmail to mcselfhost, with one account.
Basically federation works based on a block-list, not a allow-list, unless the admins of the instance set it that way, just like email providers.