I struggled through understanding how the routes work myself a few months ago, so I wrote a self answer for next time I need to work on them.

It covers the things I got confused by, and might be a way to get started and then narrow down any more specific questions.

I've now added an answer which has some overlap but is more specific to the user profile page.

Thanks for the link! From the title I didn't recognize that as being about routes, so I didn't dig deeper. Oops.

The routes were only in the answer, not the question, because I phrased the question in terms of my initial misunderstanding which didn't know that routes were the answer. I guess that's not ideal for browsing through questions, but at least this one exists now which is explicitly about routes.

Right -- questions start from the question. I think the current state is fine. Thanks for making the connection for me and future readers.