Can I log in with my actual account (of lemmy.ml) to other communities or federations or whatever they're called instead of having to create a account for every site (like lemmy.dbzer0.com)?

I’m pretty new here too, but from what I understand you’d need to create a new account if you want to log in to Lemmy.dbzer0.com - however, you can subscribe to communities on other instances from your Lemmy.ml account. Search for the other community by typing it in the search bar like this; [email protected] and then you can subscribe.

This is correct. You don’t need a user on each instance. You just subscribe to remote instances’ communities from your home instance, and interact with posts as usual.
Or in the address bar, like this: https://lemmy.ml/c/[email protected]

You can also go to https://lemmyverse.net/communities, set your home instance to lemmy.ml (or whichever you want), and it'll automatically link you to the correct place to see it on your home instance.

There's also https://addons.mozilla.org/en-US/firefox/addon/lemmy-instance-assistant/
- Hi there! Looks like you linked to a Lemmy community using a URL instead of its name, which doesn't work well for people on different instances. Try fixing it like this: [email protected]
I think it should be ! [email protected]. With the ! In the start or else it will not show up in the search

nope, but you can browse resources of other servers here on lemmy, if they're federated, list of federated/blocked instances is available under "instances" link at the bottom of each page, some instances defederate each other for some reasons, shit just happens

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

I don't think so, but you can access the communities on those other sites through lemmy.ml. Either because someone else in lemmy.ml already has or by directly searching for the community url and subscribing (thereby enabling access for everyone else on lemmy.ml).

No, the user account is bound to the instance you created it on. But you can participate in communities from other instances from your home instance. That's the whole point of federation.

Think of it like email: If you have a gmail.com address, you can only log in there, but you can send and receive mails from anywehere, not just other gmail.com users.

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)

I keep hearing conflicting reports of whether beehaw has defederated from lemmy.ml. I don't actually think this is the case. I've been posting comments in beehaw communities just fine and another user told me lemmy.ml isn't on beehaw's defederation list.

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