Question on 'Custom page Template'

Am I correct in thinking that this is a GP feature > https://prnt.sc/o7akb2

So, if correct, where is a 'generic' or standard template that I can copy and paste over and then re-name it so that it is found by the dropdown?

Should my workflow be > find page.php > rename it > page-name.php then add this:

/*
 * Template Name: My Name Of Template
 * Template Post Type: post, page, product
 */

If that is correct then at what part of the page can I edit?

For example, I would like total page width like this: https://prnt.sc/o7besv

So, my question is - where do I add the <div class="container"> custom code </div>

Thanks!