How to get started on making this mobile OS?

[maybeJosiah]

Okay, I can't find a free AArch64 ARMv8-A or later assembler (for assembly language) other than from GNU in "binutils", I got that with target of "aarch64-none-elf", anything else I should know? Also, test device might be something like "Samsung Galaxy A15 5g phone", been using grok on X a lot because there is like no guide for this, sure there are those huge reference manuals but they are huge. Also looks like needs secure boot which I can handle by cryptographically signing kernel with OpenSSL on Linux, may have to include a stripped down version in bootloader. I am still learning. At least "binutils" is free.

I also got that there is a way to do like build .bin or in release .iso, then attach bootloader unlocked phone to computer, then flash, no intermediary (except cord) involved. Pretty sure it can be done with an Ubuntu Linux machine and no bootloader interface for Android except one needed for bootloader unlock, that can be on a Windows machine. I could also root and maybe even just cut the Windows machine entirely. There are also NDAs for Qualcomm stuff, might need that. So much to learn. X E.

[maybeJosiah]

Actually, I may need Heimdall for that device and to have Android Studio and Fastboot tools but still, doable. X E.

[maybeJosiah]

Also, "Download Mode", a hardware thing that is required for flashing should stay intact as long as I just flash system and boot partitions including if I make a new file system type so I can continue trying to oblivion with this device. I may have to separate out bootloader and system for this but that is okay. X E.