You can use your World account to post in communities on a different instance, but if you want to join that instance you need to make a separate account.
It’s best you have one account on World, go to the top where it says communities, search under all, and subscribe to what appeals to you. You can still interact with the other instance with no issue.
Instances are completely separate servers. The downside is if World goes down, then you can’t do much, but you can login to a Blahaj account and still use Lemmy like normal, except access World posts. So the idea is to create your own instance and customize your feed, if a major instance is down it doesn’t impact your ability to browse instances that are still up.