Scrolling effect

Does anyone know how to achieve this effect where the shape fills the page as you scroll, but the image stays in the same place? I know how to expand the shape but am confused on how to keep the images behind it in the same spot. Also when it expands to fill the page, I’m guessing this takes you to another page maybe?? If so it would be great help on how to do this too!

What I’m trying to say will make sense with these, or find the website to see for yourself-https://andrzejdobosz.pl

thanks!!