They can be running bare metal on top of a type 1 hypervisor, on top of a UNIX, Windows, mainframe, micro, or whatever.
Android and ChromeOS, which although based on the Linux kernel, don't expose a POSIX userspace to regular app developers as public API, only OEMs get to play with it, or placing the devices into developer mode like Crostini.
It doesn't matter at all on the XBox and Nintendo, but it does on PlayStation.
Its relevancy depends pretty much how deep one still is into classical UNIX like workflows, but like COBOL and Fortran it isn't going away anytime soon.
Unysis still sells Burroughs as well.
No, Android apps can contain native code that uses Linux syscalls.
"NDK C library"
https://developer.android.com/ndk/guides/stable_apis#c_libra...
"Improving Stability with Private C/C++ Symbol Restrictions in Android N"
https://android-developers.googleblog.com/2016/06/improving-...
"Android changes for NDK developers"
https://android-developers.googleblog.com/2016/06/android-ch...
Termux developers also think they could do whatever they felt like on Android, guess what, they can't.