What is the difference between a disposable app and a blank xamarin app?

Use a blank app if you want to create custom views using native layouts for different platforms. Use a single view app if you want your view to be common across platforms. You can use both approaches in your project anyway. For example, you can use Xamarin.forms

for some data driven views and combine them with some of your own views to interact with the platform.

If you want a fast development cycle to prevent empty apps, a single viewer has the benefit of shared code across all projects, and the user interface is created once and used across all platforms.