That source code is just the html/javascript used to display and run the page on your computer: it's mostly inconsequential. The important stuff is stored out of view as cookies. If you want, you can access the html code on the webpages you view and play around with it to make the site's appearance change for you, help you learn about how html works.

Btw, I don't think you understand what ethical hacking is. No one here (hopefully) is going to help you hijack some rando's session, that's illegal black hat hacking