When and how will php code in a user made WordPress page be executed?

What happens next?

Nothing. That’s it. Loading the template is the last thing that happens.

I suppose the post loop in the template gets executed.

When the template is loaded the PHP code in it just gets run like any other PHP code.

Most templates should include a pattern of code that’s called ‘the loop’. This code does what the documentation says it does.

Where and how does any php code embedded in my web page come into play here?

You don’t embed PHP code in page content in WordPress.