By a developer perspective.
You can use Samsung Gear Watch Designer. Quite easy to use, limited by the intrinsic limitation of the program itself.
You can sell you watch faces on the Samsung Store. There's a lot of competence and the store is not well organized as Google Play Store. But Samsung watches are the most sold watches after Apple ones.Sharing revenue is 70/30.
Wear OS (formerly Android Wear).
Watch faces on Wear OS are Android apps.
You have the advantage of full control over watch hardware. But you need to be a Java programmer.
Potentially limitless. But if you're not familiar with programming, avoid it. I personally like it the most. But I had to develop several different sets of Java libraries to control sensors, location, Google Fit access etc. Gratifying, though. Sharing revenue on the Play Store is 70/30.
Apps like Facer and Watchmaker stand like an interface between the watch face and the watch hardware.
You don't need to program, just assemble graphic elements.
Also, you can develop once and deploy on multiple watch platforms, from Samsung Tizen to Google WearOS.
If you're interested in selling your creations (earn a few bucks), with WatchMaker, you can package your watch face and sell it directly on your Google Play account but it's a less popular platform with respect to Facer.
To sell your watch faces here, you have to be invited into the Partner program. Sharing revenue is unknown.
I believe pressure from global brands will increase constantly. For example, the recent EU copyright law will be fully effective in Europe in a couple of years. But,then again, you shouldn't sell copyrighted work, so...
I don't find productive playing in any sandbox, unless you do it just for fun. I want to sell what I create.
But a lot of people does find it interesting. It depends on what you want.