Libraries. If you had to write everything from scratch, nothing would ever get done. You pick a UI library/framework that meets your needs and is supported in the language you’re using, and go from there.

You can use a Web interface, it’s absolutely hideous but it gets the job done. I hear QT is pretty easy to develop for and it’s cross-platform.

The docs directory literally has a stub on getting the repo up and serving and also a note that they are cleaning up and working on the documentation https://github.com/ente-io/ente/tree/main/docs