Is there a way to know what variables are used in backend of a website that is not yours?

A friend of mine told me to reload the page and watch in the developer tools if there is something loading. I saw two files loading in the Network section every time I reload the page. But I couldn't use it to know the variables used in it. And do tell me if there is a way to know what variables are used in the backend, can I change the value of that variable? PS: I don't know which backend programming is used either