It runs basically the same PebbleOS, so they’ll work with any app that works with the original Pebbles. They plan to keep using the community app hosting at https://apps.rebble.io/. There’s also GadgetBridge that’s compatible. Eric mentioned on HN the intention for an official open source library that can be used to make other companion apps too.

Yeah the mobile app is open source too https://github.com/pebble-dev/mobile-app