That makes it sound like Linux is just a big dump of code. In reality, if you want to add some sort of device support, it's expected that your code will fit in

That makes it sound like Linux is just a big dump of code. In reality, if you want to add some sort of device support, it's expected that your code will fit in with the rest of it, e.g. use device tree and all the other infrastructure, and use it correctly. Some subsystems have stricter guidelines yet, e.g. if you want to add support for graphics hardware to DRM, the graphics subsystem, there must be a corresponding, fully featured open source userland implementation for it.