It depends! They can evolve out of existing friendships if you're both interested in that. Or some people seek them out explicitly, though this is probably harder due to a lack of channels devoted to this sort of thing.
For me, I've been generally looking for more new friends and one of them is not exactly a QPR yet, though I believe we're both interested in the prospect. Neither of us was explicitly looking for one but it's just how things evolved. We have a lot of shared interests, are both aroace and both want a QPR with someone in similar ways.