To really learn about the OS architecture, set aside a partition on your hard drive, and use debootstrap (within a chroot environment) to build a new OS from sc

To really learn about the OS architecture, set aside a partition on your hard drive, and use debootstrap (within a chroot environment) to build a new OS from scratch. Build up, from kernel to init and the barest of coreutils (busybox is an excellent substitute, with everything you need already built-in). Boot up from this partition, check the console messages, and keep going until you can get to the login prompt without any errors. You'll soon find out how the systems hangs together. (Of course, there's also Gentoo and Linux-From-Scratch, but you specifically asked about Debian. Gentoo is far better suited to this, but also takes far longer...)