Keys, like everything in digital devices, are just strings of data. So if they are on device, it’s the matter where they are stored on the device.
Google and Apple implementations are going to store them in secure TPM chip, basically once written there should be no way (people knowing darker side of TPM can disagree) to get them back. But, if I understand correctly, there is no forced way how to store them in the spec, there can just be a way Google implement it in such a way, hope they add open API to Android.