This probably has been asked a million times, so apologies in advanced. But can we log into other instances with our login here for example if I went to lemmy.world and logged in with Designate6361@lemmy.ml should this work? Or do I just need to make an account on their instance as well ? Already know how to browse and comment on other instances but this was interesting me to know if it could be done.
No, each instance has its own database of users so you’ll need a separate user for each instance. However, as you pointed out, you can subscribe to communities, comment on posts, and upvote/downvote posts on an instance with an account from any other instance that federates with it.