A good way to learn how Grav works is to look at existing code, especially Grav team plugins.
I know the form plugin can store data in the user/data folder
Grav form plugin github line 4
But I am sure there are much more, and I guess you have a lot more experience with PHP than I do.
Maybe you could help me with this?: https://discourse.getgrav.org/t/get-form-data-select-input-and-add-it-to-page-frontmatter/4465/2