Is it possible to write software for a camera or how to modify it?

Question

Programming

Is it possible to modify the OS of such cameras as canon, Nikon. If not, what are the possibilities to write everything from scratch, adding an extended list of features?

Answer 1

https://www.youtube.com/watch?v=Jrx2kJ6dBt4

Answer 2

You can use CHDK (Canon Hacker's Development Kit) .

Answer 3

Everything has long been thought up for you:
Canon - www.magiclantern.fm I personally use it. Mostly useful for videos
The installation process is quite simple:
1) Make sure that the software version matches the one specified for the custom firmware (if not, install the required one.
2) Download the ML firmware, copy it to a USB flash drive and start the software update through the menu of your carcass.
Nikon: https://nikonhacker.com/ - I don't use "yellow mouths", so I can't say anything about functionality.