Welcome to the first hint for your first contribution! Looks like you don't know where to start? No worries, we've got so many folders and files that you'd almost imagine you're falling into a rabbit hole ending up in a whole different world.
Remember our analysis after we determined the issue? Let's take a look again:
It's about users.
We need to add a field to the profile.
It's a field for the shoe size.
The field should only accept values from 39 until 47.
It can be empty.
It should be editable on the admin member edit page.
It should be visible on the public user profiles.
We're focusing on the first bullet: It's about users. As noted in the overview we have several important apps:
This is the entry point of the system, it holds everything together.
Basically everything related to the association member administration.
Holds the code to manage boards, committees and their permissions.
Now you should be able to find the right folder. It's basically in the list above ;)