I just search for them on my home instance, subscribe, and then comment/vote from my home instance. No reason to sign up everywhere.

No, that is not possible, each server (instance, like Lemmy.ml, lemme.ee, Lemmy.world) is completely on its own. The reason you can see posts from instance x on instance y is because they sync posts and comments with eachother. The are what is called “federated”. Some instances unfederate other instances. Then the sync is only one way. Basically saying “I don’t want to see your posts and comments anymore instances x, I will no longer sync your stuff to my instance”. Note that other instances can still sync posts and comments from the one that unfederates. Take for example that beehaw.org unfederated lemmy.world. Users from Lemmy.world can still post on beehaw.org communities but beehaw will not see those posts anymore.

Sort off. No need to create an account on every site as you can subscribe to and contribute to communities on other servers though lemmy.ml

No, but normally you shouldn’t have to. You can still see and interact with all communities on the instances your instance is federated with. You’d only need to create a new account on an instance that has been defederated from your “home” instance (for example you account is on lemmy.ml but you want to interact with beehaw.org)