Can anyone explain briefly how website builder create the backend html & css code.

Answer 1

Theres code saved for every element and then it gets inserted with the right colors and position and other user-changable things. CSS is often not directly saved on your site but loaded from another url and only one stylesheet on your url overwrites the colors and stuff.

How is it about JS?
Well, basically the same as CSS. Stored on another url and then loaded in.