Undertanding 'hosted apps'

Hello – still very new to blockstack but I’m getting there.
A question on hosted apps like https://blockstack-todos.appartisan.com/

I’m just going to make a couple assumptions here:

  • this app is hosed on a .com and therefore needs a server to host

  • The app environment is still running blockstack.js so all the libraries, etc are installed and running on the server host

So is it fair to say that a standard web application hosted at my own URL could integrate quickly into blockstack ‘simply’ by swapping out authentication and storage with blockstack as opposed to a local session + database?

I’ve read a bit about BNS but don’t see where that comes into play here.

while I’m on this thread – I’m assuming through the BNS, each application gets its own unique ID – where does that registration happen? thanks