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 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).