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I'd say most Linux installations today are Android mobiles. Do those have Minix too?

No, but Android does use a different microkernel (LK) as it's bootloader.

It's also likely that Google will replace Linux on Android with Fuschia, yet another microkernel OS, derived from LK.

The Linux kernel is irrelevant for Android userspace as it isn't actually exposed in public APIs.

You might access Linux syscalls and certain paths, but the only guarantee is that it will work on your own device.