this gets asked a lot. Yes, the whole point of this platform is to be decentralized. Nothing stops you, or anyone, from following 5 different communities on the same topic. The advantages are many. Basically each instance will be able to mod with/apply different rules to their communities, they will attract different crowds (even if they overlap a lot) and they will have a different style. If one goes down, which is likely in these platforms, or if say an instance cannot be sustained anymore, or if one of the instances defederates your instance, or whatever- not all is lost. You still have all the other communities to follow.
there is a popular saying that goes, ''don't put all your eggs in the same basket''.
oh I didn't mean it that way, no need to apologize. I just notice this question comes up a lot inside different communities, people often wonder why there are also two/three/N other communities and why don't they all merge. It highlights a lot of people still expect things to be centralized even in a decentralized space. But, it's good that you ask, so hopefully more people understand better why it's good thing to avoid monopolies on topics.
The search function should function the same way as the search function on the official web interface. That means however, unless you're searching the url of a specific community, it can only search for communities, comments and posts on instances it has cached. If it hasn't had a member visit a community at least once, it won't know it exists.