What knowledge is needed to write custom firmware?

Question

Android

Let's take a stock firmware from any phone and change something in it to your own, what and where knowledge is needed? Simply put, how to make custom firmware?

Answer 1

Write or collect?
Most likely the second.
To do this, it is enough to carefully read the instructions, which are just a pack on the same w3bsit3-dns.com and be able to apply them in action.