The purpose of a sticky footer is that it “sticks” to the bottom of the browser window. But not always, if there is enough content on the page to push the footer lower, it still does that. But if the content on the page is short, a sticky footer will still hang to the bottom of the browser window.
As always, Chris Coyier does an excellent job of explaining how to achieve a desired layout. It’s a shame that option 5 (CSS grid layout) is not currently well supported as it looks the best by far. Also, checkout the comments for additional solutions.