Who exactly sends requests to the backend on the client side?

On the client side, requests to the backend are usually sent by code responsible for server interaction — these can be:

• JavaScript functions using fetch, XMLHttpRequest, or libraries like Axios.
• In SPA frameworks (React, Vue, Angular) — services or hooks that encapsulate request logic.

For example, in React, a separate API service is often created:

// api.js
export function fetchUserData(userId) {
  return fetch(`/api/users/${userId}`).then(res => res.json());
}

And it is called from components or effects. Thus, it is the client-side code (JavaScript) that initiates requests to the server.