Important to say, I mean products you use even though there are alternatives, not monopolies like YouTube.
I don’t think this is quite true when it comes to YouTube. There are plenty of alternatives that content creators could use (and some do), from Vimeo to Nebula to PeerTube.
The reason content creators prefer to upload on YouTube are the user base and monetization opportunities. They’re not forced to do so, and in fact YouTube is facing competition from e.g. Twitch and TikTok.
You definitely shouldn’t need to do that, one account is enough.
Maybe you’re confused because communities can be on different instances (servers). But you don’t need to make an account on those instances, because all the instances are federated together. That means you can just have your account on one instance and follow and participate in all your communities from there.