In web programming, how is the link created between frontend and backend??

Question

Fullstack web programming

Answer 1

It is called API. requests from server to client and vice versa
Also there are websockets for sending messages from client to server and vice versa
and some other

Answer 2

2 common scenarios
1. backend preprocess the frontend before sending back to client
2. frontend comunicating with backend on realtime using ajax or other technologies