Well, you can access the AMP version of the page from anywhere, not just Google search results. I could see apps that use an in-app browser serving the AMP version of a page, instead of the full version, for instance. I guess we'll see?
I'm aware that it's technically possible but was realizing that, as a fairly active mobile web user, I had never noticed AMP in use except for Google search results.
i could see other apps that use a native browser preferring the AMP version. something like yelp, that lets you click through to the source page, for instance.
the top banner is something that Google adds, and isn't, strictly speaking, part of AMP.