How ViewModelStores.of works for https://medium.com/@josep.rodriguez/internals-of-android-architecture-components-part-i-the-viewmodel-d893e362a0d9

ViewModelStoresOf.java

public static ViewModelStore of(@NonNull Fragment fragment) {
    if (fragment instanceof ViewModelStoreOwner) {
        return ((ViewModelStoreOwner) fragment).getViewModelStore();
    }
    return holderFragmentFor(fragment).getViewModelStore();
}

...

public static HolderFragment holderFragmentFor(Fragment fragment) {
    return sHolderFragmentManager.holderFragmentFor(fragment);
}