Apple watch apps
How can I add a stopwatch app to my apple watch?
Apple Watch Series 2, watchOS 6
Add a Stopwatch Complication to your Watch Face. Watch Faces which permit adding a Stopwatch can be found in this link --> Apple Watch faces and their features - Apple Support (CA)
If you are unsure how to do that, this link explains how to customize watch faces --> Customize the watch face - Apple Support (CA)