It is open sourcing the repebble app here: https://repebble.com/app

Looking through the code, most calls go to repebble.com, rebble.com and rebble.io domains. Most of it seems to be for login, firmware downloads, settings synchronization, weather forecasts and language packs.

There are also some calls to memfault.com for crash analytics.

Most of it seems pretty innocent. If you don't like this app you can also use Gadgetbridge as an alternative.