What is the link between backend and frontend?

Question

Backend

Good afternoon!
1) Please explain, as far as possible in more detail using the example of some web application, what is the link between the backend and frontend? I need to understand what "technology" helps these two parts of the work interact? If not difficult to explain on some classic example.
2) i.e. Do I understand correctly that if we remove the database and all sorts of computational operations "under the hood", then in fact we will get only the frontend and as a result a regular website, and not a web application?

Answer 1

Data channel.
And, usually, this is the specification of the http protocol (but not always).

Answer 2

In order to be as detailed as possible about the essence of the issue, let's look at a specific example.
For example, there is an html page on which there is a regular form with several fields and a button that sends data. Let's say the client filled in the fields and clicked on this button. It turns out that a POST request has occurred, the action is naturally indicated in the form, which transfers data processing control to some servlet, and the latter, in turn, performs some calculations, generates another page and sends it to the client, everything seems to be clear so far.
Now the questions are:
1) If, for example, the backend is written in Java, then how does java understand the data entered in the form on the site?
Through templates? Or is that what web sockets are for?
2) There are many hostings on the Internet to deploy your site. Will I be able to deploy my server on their server? Let's say Nginx was recommended to me above. I just don't understand, is it possible to deploy a server on top of another server? After all, these hostings themselves work on servers.