A click turns a URL into an HTTP request, and the server answers with a status code. A cookie is the wristband the server hands you so stateless HTTP can recognize you. By the end you can find one request in DevTools' Network panel and point to its URL, method and cookie. The examples only hit your own machine.
SSE sends text/event-stream over ordinary HTTP, with browser reconnection and Last-Event-ID; simple transport is not durable unless the server retains events behind that cursor.
An agent should not open a browser for every web task: route first to Search or Fetch, then escalate on explicit signals such as status codes, weak content, JavaScript shells, authentication, or challenge pages, with retry, budget, cache, deduplication, and provenance constraints at every step.