Here is a probably not at all comprehensive list of open source smart watches/OS's you can check out.
- PineWatch https://pine64.org/devices/pinetime/
- ZSWatch https://zswatch.dev/
- AsteroidOS https://asteroidos.org/
- Watchy https://watchy.sqfmi.com/
- Bangle.js https://banglejs.com/
- (Re)Pebble https://store.repebble.com/
- Sensorwatch https://www.sensorwatch.net/ (board swap for the classic Casio F-91W wristwatch. Hackable but comparatively basic)