Why did the frontend stop going to the backend after adding SSL?

Question

Digital certificates

There is a front-end on Angular 7. It is compiled under production and uploaded to regular hosting.
This frontend accesses the backend via REST, which runs on a separate cloud server. The server is written in Java (Spring Boot).

Added SSL to the frontend.
When I go to the frontend via HTTP, there are no problems, it accesses the backend, gives me the information I need.

However, when I try to go through https://my_site , I get to the frontend, but the connection with the backend stops working.

It turns out that if I go to the site via HTTPS, then it should also automatically encrypt requests for my backend, and therefore the server does not perceive anything from the front?

Tell me, in which direction to look, what to read?
How does encryption work in general?

As far as I understand, when I visit my site, I actually download several large files, which further process my clicks, make requests to the server, and display the results.
And since I'm like, as I went via HTTPS, then requests should also be sent to HTTPS. And it turns out that on the server I have to register the public key of my frontend in order to be able to accept SSL requests...