How are firmware made?

Question

Firmware

I'm wondering how they make alternative firmware for android, routers? Where to read about it?

Answer 1

Firmware is usually one or more file system images. Alternative - most often the files in these images are simply changed. Most of the images can be mounted and edited very easily in the Linux OS family.
Start with this:
bootloader - it's better not to touch
kernell - it's also better not to touch
initramfs - here are scripts for loading and connecting other partitions.
system volume - just a section where all applications of the android operating system,
then all these sections are collected in one file - the format of which depends on what to flash and can be encrypted or digitally signed.