I’m wondering if the designer files of the premade watch caves are somewhere online so that I can use the designer software to add buttons and change colors
If you mean the watchfaces preinstalled in watch, then no. Those are not available as templates. You would have to make your own version from scratch. There are different templates included more simple to demonstrate how the software works.
There are some very nice examples of watch faces which come with GWD as samples,
they look pretty simple, but some of them are very good to learn from.