Answer 1

If I recall correctly, on Windows click ‘All Supported Files’ (or something like that) and then type your filename with extension, like ‘website.html’

Answer 2

Can you tell what software you use for writing the code? there may be difference on how to do just that in different softwares. Add that information to the Description section of original post above so people can quickly understand and offer you a hand.

Answer 3

For html .html
For Css .css
For JS .js