Matt from the Capiche team here—thanks!
Capiche's community is custom built in-house by @forouzani and @sarah, powered by Rails plus a bunch of integrations, including Customer.io and Twilio for notifications, Algolia for search, Segment to move data around, and more.
If you're wanting to build your own community, there have been a number of discussions on Capiche about the best platform to build a community, including one earlier this month. Those saw people recommend Bevy, Tribe, Mighty Networks, Circle, Commsor, and Discourse—seems Circle and Tribe are the most popular of those now. The "best" depends on the features you need.