There's a bug right now that search doesn't work if you haven't selected an instance you're logged into. I don't have a lemmy.world account, so the default selection would never search.
Once I selected my home instance to do the search tho, you'll see whatever communities come back under the relevant tab.
I have accounts on each of the instances that I browse.
The problem with search is that it seems to just return posts and communities from everything federated with the instance you are searching on.
So if I search for a specific community name I'm going to get tons of posts from tons of other communities mixed in, and I can't for the life of me find the community I'm trying to go to.