How to handle form submissions?
Doing it through a plugin would be the easiest way.

https://docs.craftcms.com/v3/extend/plugin-guide.html

Your plugin would have a controller that the form would post to, then you'd grab the data from POST and do whatever you want with it.

A good example of this you can look at is the Guest Entries plugin, that allows "anonymous" front-end entry submissions.