How to: div fills the screen

Hello everyone!

I’d like to do what in many website happens, such in these examples:

    Once the home page is loaded, the bottom text is always shown even if the browser size window is modified (desktop format) till the user scrolls down.
    It happens also here even in smatphone format (+ it requires to be “scrolled more”).

It looks like thee’s a “div” in html+css which adapt itself to the screen until scrolling down.

I’m wondering how to do this without css because I’m not use to it.

Thank you!