I mean when anybody can check how backend work, but nobody can change the backend.
That's for purpose to prove users that there is no any cheating on server.
When you say

when anybody can check how backend work

do you mean anyone can look at the code of the backend? That's certainly possible, and it can be done without compromising the security of the application.
There are plenty of secure backends and libraries that are published in open source repositories.
Assuming that you don't share private keys or other sensitive data, and your backend is built with modern security principles in mind, it should be possible to allow others to read the backend without risking the safety of your application/data.
Is it possible to create OPEN BACKEND? for example written on python