Why can backend proxies cross domains?

Question

Why can the back-end proxy settings be cross-domain?
Don’t quite understand

Answer 1

The same-origin policy is the browser's security policy and is not part of the HTTP protocol.

When calling the HTTP interface on the server side, it only uses the HTTP protocol and does not execute JS scripts. There is no need for a same-origin policy, so there is no cross-over problem.

Answer 2

Why is there cross-domain? It is the security policy of the browser. Without the browser, there would be no cross-domain.

Answer 3

Cross-domain is originally the same origin strategy of js