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)?
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.