Answer 1

How could you think that someone could explain a topic that you feel difficult in details in a post limited to 2048 char? :o

This kind of general questions about such topic have not really their place here: if the sololearn course isn't enough for you to understand, you will be better advised to be autonomous ans search by your own on internet... it doesn't lacks of such ressources, and offers enough differents sources for finding one good suited for you ^^

Answer 2

I know but still you can't change what it is ,but thankfully it's quite initutive to understand what getElementById means and all that , hopefully this link helps since I found it useful when I was trying to understand how html DOM works
https://javascript.info/dom-navigation