A couple years ago I became extremely interested in the ability to dualboot some older iPhones.  Occasionally a prototype iPhone would have its NAND dumped, and it was possible to install its OS onto retail devices. 

Doing this involved decrypting disk images, paritioning the iPhones NAND, and booting custom ramdisks. 

The iOS bootloader also needed to be patched, which was done manually via a disassembler program. Patches needed to be made to bypass signature checks and ensure that the bootloader was booting from the right partition.

I also find OS development very interesting. A couple years ago I created a shell script to crate a bootable Linux ISO compiled from scratch that was about ~10MB in size. It can very easily be scaled to include different kernel configs and additional binaries. 

https://github.com/Rymu74/busybox-iso