If you search on either a lemmy or kbin instances, it should be able to find comments/threads on communities/magazines on Threadiverse (lemmy/kbin) instances that (a) are federated and either are local to the instance or (b) have a local user who subscribes to the community/magazine in question.

Neither will typically search the entire Threadiverse, because they only search content that they have a copy of locally, and no one server intrinsically gets a copy of all content on the Threadiverse.

I don’t know how kbin deals with searching for comments off Mastodon. That’s a good question. You can set a magazine to include comments with given hashtags, and I am wondering if basically it sees the content on local magazines or remote magazines with the same rule above as having a local user who subscribes to such a magazine.