Android already runs on 1 Billion devices which is basically Linux…
At which point? It takes only the Linux kernel. But kernel is the least important part from practical usage perspective. Everything else is different, the bootloader (no GRUB or systemd-boot), screen compositor, sound system (not a Pipewire or PulseAudio), package format, init process, shell, even the standard C library (Bionic instead of glibc).
There are projects to run Linux on phones (see: wiki.postmarketos.org/wiki/Devices), but it takes huge about of reverse engineering and work.