As stupid as it may sound, I admit there are things about lemmy that I don’t understand. One of those things is the separation to .ml, .world and so on. I just came across a post on lemmy.world but it reads I haven’t logged in. So: do I have to create a new account in order to subscribe/comment on .world or can I use my current account from .ml?
You can use your Lemmy.ml account! You may wish to create another account if you want to, say, browse just Lemmy.world (or any other instance), but you can also find that same post from your Lemmy.ml account by searching for it. What happened is you clicked a link to the Lemmy.world version of that Lemmy.world post, not the way Lemmy.ml sees it.