You can see yourself most of the requests a browser makes in its developer tools. (The requests of the TCP handshake and sometimes preflight requests aren't shown, but you don't need to bother about that as a beginner). For example, if you visit https://example.com/ the browser makes a HTTPS GET request and in the case of a success receives a HTTPS response with the HTML document and a HTTP status code 200. You can see both the request and response as well as headers in the network tab of the developer tools.
To open the developer tools press F12. Afterwards click in the network tab and reload the page. On the simple web page https://example.com/ one entry should show up on another web page usually multiple entries show up, e.g. for each image, CSS, JS, font, etc. file. Click on an entry to show additional information like the headers of the request and response.
You can read more about the Network tab in the Chrome developer tools here (the network tab of other browsers is very similar):
https://developer.chrome.com/docs/devtools/network/
More Advanced Use Cases
With tools like Fiddler or Wireshark you can inspect even more network traffic. This can be useful when dealing with more advanced scenarios like debugging authentication, CORS or NTLM (challenge response) issues. Or figuring out if different devices in a local network can reach each other.
With Tools like Postman, Insomnia or hoppscotch (open source on GitHub) you can fabricate not only GET requests but also other requests (POST, PUT, DELETE, etc.) and see the response data. This is very useful when you are working with REST APIs. Some Code editors also have extensions for making such requests, e.g. the VS Code extension REST client. Alternatively you can use browser extensions like Servistate or RESTer.
If you need a REST API to fill your application with some data for quick prototyping you can use one of the following options: