Developer Reference request

Hello guys!! Could anyone give me a very more detail reference about frappe apps? I have found just some basic documents on the internet. For example, I want to customize menu or menu item. Hope you guys will help.

This is the best resource right now:

https://frappe.io/tutorial

Thanks!! I already checked it out but it doesn’t give me enough. I want to customize the views for example, POS and I also want detail of the structure.