Co-founder matching is a thing too (ex: https://www.ycombinator.com/cofounder-matching), but unsure how far it works.

Famously, the GitHub co-founders and the Instagram co-founders met at co-working spaces / hacker meetups. Tom Preston-Werner gave an interesting talk on how to build successful co-founding teams and products: https://www.youtube.com/watch?v=mGTpU5XUAA8