Previous watches used 100% own made OS called Tizen OS and that did not support google apps and other apps through Google play store. So then samsung joined Google to build a WearOS based on tizen and WearOS so watch can support both apps from samsung made for Tizen and apps from Google made for WearOS.
This was also so that Samsung didn't have to re-build their apps to work for WearOS, instead galaxy watch uses Tizen made Apps from Samsung.
This is what i read, even Google assistance is own made version to work on WearOS for galaxy watch.