As I understand about the amp, the amp is only for mobile devices. If I'm using responsive design, my web server provides same HTML document per page for every device.
But If I want to use the AMP, the web server should decide whether it provide HTML for desktop or it provide HTML for AMP.
How do I do that? By User-Agent?(I think it is ugly and not scalable)
I missed something?
You should have two urls for the same content. The main article would be http://example.com/my-article and the amp version : http://example.com/my-article.amp
there's no user-agent switching for the same url. You just have to specify in the main article that you have a amp version of the document with a link tag :
<link rel="amphtml" href="http://example.com/my-article.amp" >
and in the amp version specify the address of the main content with a canonical :
<link rel="canonical" href="http://example.com/my-article" >
To answer your question, the actual "mobile detection" is sort of done in the search results page as it is google who will determine what link to show to the user.