Where is the page.php coming from in Atahualpa?

I want to create an alternate page template.

There is no page.php in this theme. You can create a custom template by copying index.php. I've made several.

I asked a similar question just a few minutes ago. If/when we create new templates by creating index.php, what is the impact in repeating code and, do we just handle a theme upgrade manually for custom templates?

if you make a copy of index.php and then name it page.php, you can make whatever changes you like. Usually you'll want to find the loop and keep it in, but you'll probably modify it. I recommend reading the documentation at wordpress.org in the codex.

and, yes, when you update the theme, you'll have to manually update you customized pages.