The Apple human interface guidelines are a good source of information:
https://developer.apple.com/design/human-interface-guideline...If you want something that isn't tied to Apple you can find a list of similar guidelines here: https://en.wikipedia.org/wiki/Human_interface_guidelines#Exa...
Personally I find that KDE, gnome and xfce have great UI, you can find their guidelines in the list above.
Regarding implementation I'm a big fan of Qt, there's an excellent python binding as well (PyQt), which makes it easy to get started especially if you already have your logic implementing in Python.