Answer 1

Consider them as extra elements from the current element, with relative position to current element.

Here a sample usage

https://code.sololearn.com/W7hmu7GdkCsy/?ref=app

Answer 2

It's the pseudo element of CSS.

SoloLearn:
➡https://www.sololearn.com/learn/CSS/2257/

MDN:
➡https://developer.mozilla.org/en-US/docs/Web/CSS/::before
➡https://developer.mozilla.org/en-US/docs/Web/CSS/::after