I’d like to improve my mental model of how the browser goes from downloading index.html, app.js, and vendor.js to running the code I write in routes/application.js. A lot of the work between these two points is obscured by bundling.
Where do I start with understanding this? Or can someone who does understand help?
Yes the metal model is hard at first but later you don’t know why it was hard 
Maybe I can try to answer your questions. First mental model on routes is to understand that a webserver like Apache serves the index.html file created by ember-cli on all routes. See source:
https://bloggr.exmer.com/about
https://bloggr.exmer.com/authors
Ember renders on screen what you see depending on the things in the routes/templates etc. Hope this helps a little.