What is it that any website has? A contact form. And lots more, but let's focus on the contact form. Or a form to get a quote, sample, presentation, membership, whatever. With that in mind, and with the Userprofile fieldtypes made, I realized that a form creating extension is what was next.
All I had to was attach the already made fieldtypes to a form object, and store it the same way as the Userprofile. And add a mail functionality to confirm the submission. And... Of course things got a lot more complicated than I though at first. But with as little previous knowlegde I had on Symfony apps and the Vue UI, I never got stuck in getting the code to work. My coding mantra of "copy what they did" also applied here. The code of the Pagekit core is easy to read, and most basic things have been done in the core. It's just a matter of finding where they did it, and apply the same on your extension.