Bear with me, I am a very junior R person (but excessively senior with BI tools and SQL)

Your inputs are….
bikes_london, which literally has each straight line from point A to point B with the count of cycles hired (which controls line weight). My equivalent to this would be a file of routes, likely replacing count with a route ID and having different colors per route.

I would also need a similar file if I wanted to plot roads

Shapefiles for water and buildings – silly question, where do you get such files? I would be looking for US data.

Does the order of the layers matter? It looks like they are specified from the top down.

Once again, this is a fantastic map.