mobile app

I need a DEV to help me in PHP understanding how to solve a problem to scrap contents from a web page that use Js load content. I don’t need you to make the code but to help me understanding how to do it. I made a PHP from scratch code and i can get every contents from the distant webpage, but i can not understand how to get access to contents that are loaded by JS inside the page. CRON TASK / SCRIP PHP FROM SCRATCH (no Framework) NO CODE TO MAKE JUST HELP ME UNDERTSANDING HOW TO DO IT