https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout
https://developer.mozilla.org/en-US/docs/Web/CSS/flex
To position an element "fixed" relative to a parent element,
you want position:absolute on the child element, and any position mode other than the default or static on your parent element.
#parentDiv { position:relative; }
#childDiv { position:absolute; left:50px; top:20px; }
This will position childDiv element 50 pixels left and 20 pixels down relative to parentDiv's position.
To position an element "fixed" relative to the window, you want position:fixed,