What app are you using? If you’re on iOS, I know the Voyager app allows for selection of filtering your search to Communities, Users, Posts, etc. and Lemma has a pretty good smart search that has found the communities I was looking for.

As far as I am aware, at least one user on your instance needs to be subscribed to a community on a different instance or at least specially searched for it to be searchable. In your case, if no one on lemmy.world is subscribed to or specifically searched for lemmy.radio/c/amateur_radio, you wouldn’t see it on a search from you lemmy.world account. Otherwise you’ll need to use a browser to find the community and seek it out on the web. Not super streamlined yet, I agree.

The only other more functional browser searches that I know of are:

lemmyverse.net and browse.feddit.de