I do not have any intentions of selling my watch face. I want to wear it for personal use.
If your watch is Galaxy Watch 4 or newer you use Watch Face Studio, I assume that is the case and put the question here.
If you have Galaxy Watch 3 or older then you use Galaxy Watch Studio and generate a TPK file as it is Tizen not Android.
While you can generate an APK or an AAB executable file (In the Build Settings) generally withWatch Face Studio you generate an .aab file format or just run on device without the build.
See this Guide for testing On Physical Device
Hope this helps,
Samsung Developer Relations