SPFX Extensions deployment/scope questions

[Brent Ellis]

I am starting to explore building SPFX extension (with eventual goal to do a menu in the header for a site collection).  I've followed the HelloWorld intro and deployed to my Dev tenant.

Is it documented anywhere yet how to make it scoped to a Site (Site Collection) as opposed to a Web (individual site)?

So based on my reading, it gets packaged and installed as an actual App.  In the event I do scope it to a Site Collection, does it matter which site the App is loaded to to activate the CustomAction?

Meaning if I install it on a subsite, will the extension get applied to the root site?

[#2]

I think not. framework is currently making use of the 'add-in' deployment model which is well explained here https://blogs.msdn.microsoft.com/vesku/2015/11/20/automating-add-inapp-installation-to-sharepoint-si...
if you want to push the app to multiple sites you will need to write some automation to do that