Answer 1

Have you tried using the search bar or doing a simple google search for the syntax?

Answer 2

https://www.sololearn.com/discuss/2866976/?ref=app

Answer 3

It is easy.
If you want to use the user input in the same site watch this video,
https://www.youtube.com/watch?v=KB6Yg5hNrqc&ab_channel=KeithPaterson
You will pretty much get the idea.

If you want to send input to other files you would need to learn how modules in JS work. For that have a look at this,
https://www.w3schools.com/js/js_modules.asp