A place to start is to take a look at a website.
One of the most useful and probably most underrated hacker tools that there is at all is the browser devtools. To open devtools while your browser is on the viewport use the following command
/* Mac open whatever panel used last: Command + Option + I open console panel: Comand + Option + I Open element panel: Command + Shift + C Windows open whatever panel used last: Control + Shift + I open console panel: Control + Shift + j Open element panel: Control + Shift + C */
We can right click on any of these files and open it up in a source folder
we can see a single long line of code that doesn't make a whole lot of sense. But we can prettify this code to make sense to us by using the curly buttons in the source folder
xhr is xml http request. It a name that came to be known as
we can check the initiator of the request by using the initiator tab and pretifying the code
The line of code is the one that initiated this call. When this is done you can see that there are API calls being made. You can use this knowledge to check for more API calls.
Headers can also be checked for. From the header tap we see there is a API call being made to a URL.
The dev tools also give a search functionality. We can search for the API calls there and we get a pretty quick response.