A very very simple simulation of the very simplest version of the light controlled motor circuit. One of these on the left and one on the right. The photoresistor, with tape over it to keep it from activating the touch sensors in the screen, is placed facing the block displayed on the cell phone screen. When the block is white, the motor moves. When it's black, the motor stops. 

You can play with it here:

https://www.tinkercad.com/things/aQwsvZ6Vvcn-spectacular-fulffy/editel?sharecode=O67i2i5PQV1QD5hZTAP_-gB1w6oQcbMkimeN0p2bH6E

Limitations:
1. Can't go backward. (A more complex circuit might do that)
2. No speed regulation. (But the phone can use the compass / IMU to stay on course)
3. Very limited power (resistive drop through transistor, meh)

Note that the batteries shown here are just a sample. AA's or a USB battery are probably a less costly option. 

KEY POINT: The cell phone controls the motor from a web page (or whatever) which has access to the phone sensors, the internet, javascript, etc... The web page can be served from any free host, like github. The javascript on that page can send sensor data to services, get back complex analysis, directions, etc... You can do voice, image, teleoperation, GPS, etc... With an old smartphone and a few dollars worth of electronics.