It doesn’t seem so technically difficult if you can get the transit data. The phone GPS supplies your current location for the bus version, or alternatively you can just identify the most recent stop you saw. San Francisco has around 1000 city buses (I looked that up a while back) and you can put fairly precise coordinates, bus number, and route number of a bus into 4 or 5 bytes. So the complete locations of all the buses is around 20K which is less data than the javascript bloat that nextbus.com sends on every new visit. You do want the route maps (from transit agency) and street maps (openstreetmap) but overall it’s not huge.