AXML: Describe the UI for only Android apps. These are the same files used by Android native applications.
XAML: Specifies Xamarin.Forms layouts; these files describe a cross platform layout that each platform then renders in their native controls
2 - Can xaml designs be applied in axml?
Xaml designs cannot be directly copied into AXML (as they specify different controls) but they can be ported to AXML. Conceptually, xaml files describe how a UI should look on both platforms.
3 - How do I apply my css to axml files?
Cascading style sheets (CSS) are not applicable to Xaml as they describe how to style HTML. To style Xaml, look at the styles API used by Xamarin.Forms.