How to create custom firmware for IoT?

Question

Firmware

Please tell me how to approach this issue. I would like to create some non-standard applications, use different devices, such as fitness trackers, smart watches, players, not quite for their intended purpose. What to read in order to understand how to shove code into it that would solve my problems?

Answer 1

Reverse engineering off-the-shelf devices can be quite tricky, to begin with, due to the fact that devices for the end customer may come to retail with blocked/missing ports.
Better start with a custom application for a universal open device like arduino.