Can I Use WordPress as a Back-End

Hey guys, I have a frequent question about WordPress and Web Design. I make websites using HTML, CSS and JavaScript / Bootstrap 4, SASS and etc. But not the backend end, you could use WordPress as your backend. For example, I make the form in HTML, there is a plugin that sends all the data typed directly to my email. Or another example, a login area, could use WORDpRESS to create a restricted area of students or etc? Thank you.