A system for accepting applications from sites, with setting up form fields?

Question

API

Good afternoon.
There are 4 different sites for buying cars, the sites have forms for accepting applications from visitors.
We need a system for accepting applications from different sites for buying cars. With the ability to customize your fields in the form, including fields of the "file" type for uploading photos, it is possible that through some kind of API, the data will be retrieved and entered into a single database.
Services, CMS plugins, etc. are welcome.

Answer 1

What kind of CMS is that? The usual form - filled out, attached a file and sent? Setting fields through the admin panel in the CMS? And it is not clear what kind of a single database where applications are received ...