To answer the question of the title promptly, DTube isn't actually involved with the current search used for the site. That's done by @thekyle at AskSteem. The reason to why is because it's already a time consuming job to make DTube. Search is a very hard thing to do right so until the dev team is ready to push onto search, that task is handled by AskSteem.

To answer the other question, how would you determine what is more relevant when asking a question. You'd have to make a formula or an algorithm to do that. DTube itself is decentralised and can be more decentralised as more people utilise on local IPFS nodes but for now, the search is not in they're hands but is operated via an API if I'm correct. In the case of search, it is inherent to have an algorithm. I do hope there is a way to make this as open as possible. I hope these answers help you.