Linux

Thank you for telling that, I absolutely had no idea about that. In my mind, it has been a almost brute force reverse engineering effect, with some help from some documentation (for example, reference metal docs) or having some open source stuff for apple stuff (if that even exists).

regarding bootloader, is it not the case that bootloader just checks for signature of os, and it does not allow you to boot anything else. I did not mean that having a bootloader unlockable means having docs, but as i get it, the general approach to get android image working is to load a gsi (generic system image), if that does not work, we swap kernel or some other system stuff from available os images (which are closed black boxes mostly). now if we can not even boot a gsi (or some other android tree for lineage os), then there is no hope in running anything. and even if gsi runs, that may still have broken stuff (eg, camera or wifi, which i know are some of common culprits).