How to create new page template in any upfront theme

I am actively "playing" with the new Upfront themes on my local server testbed. Of the three I think I like Scribe the best for what I am trying to achieve with my genealogy site but it seems to be a bit buggier than the other two. In general, I am LOVING the customizer -- I've tried several and this is by far the most user friendly I've tried. That being said...

I am somewhat confused by the template structure and displaying page content. When I am editing or adding a new page in the dashboard, I want to select a template, but when I do, my page doesn't display the content. (Well it did in Scribe, but I can't seem to get it to work in Spirit.) If I want to create a new template that will show up in the dropdown for pages, how do I do that?

Is there any more information about the layout/template structure available?