Follow one click all the way down

A visitor types your domain. DNS turns the name into an address. The browser opens a TLS connection to that address and checks the certificate. It sends a request. Your host routes it to your backend.

The backend checks the session cookie, decides who this is, asks the database for the rows this person may see, and returns HTML or JSON. The browser renders it and runs your JavaScript bundle, which calls more API endpoints for the rest.