My goal is to ultimately create a ready-to-flash SD card .img file of Raspbian that contains custom programs/packages built in so that when I boot up the Pi, it'll automatically run my programs, show custom splash screen, etc. Doing some research, I think building on top of Pi-Gen is the way to go (correct me if i'm wrong).
But before doing that, I'd like to learn the building blocks that would help me understand how to do this rather than being told how.
Is there any specific material that anyone would recommend reading to understand how the Debian OS/packaging/file system/boot up process/etc. works that'll help me with this task.
Currently reading: Debian GNU/Linux Installation guide and Debian boot process material on the web