what is the difference usability between less, sass, scss and css ?, and where is the best place to learn sass, scss, and less?

Answer 1

I would advise you to learn scss first.
You could learn scss from the official site https://sass-lang.com/guide

You could use the following scss template to test out your codes later.
https://code.sololearn.com/WUDrkli0UntQ/?ref=app