This is not really / fully “virtualisation”, rather “containerisation” or classically (this is the correct term) “para-virtualisation”, because host and guest run on the same kernel. Hence the Android guest must support the kernel version the SailfishOS host runs on, which is (on most Android devices) an Android vendor kernel.
I concur with @attah (though he phrased it a bit awkwardly) that we should clearly differentiate between this “Android base” SailfishOS runs on (besides the kernel, there is also bootloader etc.), which is not the primary topic of this thread, and the para-virtualised Android, which provides an Android runtime environment on top of SailfishOS (which Jolla markets as “Android App Support” and technically is still called Alien Dalvik, although it is and always has been much more than just the Dalvik JVM), in order not avoid confusion among less technically savvy readers.