>but can you tell me which of the (php) files contain the custom email template structure..
there is no single php file that does this, this is pretty much all done dynamically by using filters
>Also, can I customize the “Thank you for ordering” page..
if you thank you mesaeg is too small, then it’s probably your theme’s css that sets this, so you should look there and overwrite as required
> in V3 this info appears below the form
no, it does not (not bey default anyway)
you can use any of the demos here https://demo.wp-pizza.com/ and you will see that it shows the page title, some text you set in localization, and then the order details (if that has not been disabled )