How are static pages populated in WordPress, in a custom theme?

Question

Layout

New to WordPress, so I don't get it:
When we create a theme, we create a separate template for each page.
For example, for the blog page - index.php
If this is the main page - front-page.php and for them we create separate designs (for the blog and the main one)

And then there are the pages Contacts, About us, Gallery ... and their design obviously should differ, but they make one page.php template

Question: How are all these pages (page.php) filled? How are they all designed differently?