DoToo.Android

This is the Android app. It only has one file:

The important file here is MainActivity.cs. This contains the entry point for our application if we run the app on an Android device. The entry point method for an Android app is OnCreate(...). 

If you open MainActivity.cs and examine the OnCreate(...) method, it should look something as follows:

protected override void OnCreate(Bundle savedInstanceState)
{
    TabLayoutResource = Resource.Layout.Tabbar;
    ToolbarResource = Resource.Layout.Toolbar;

    base.OnCreate(bundle);

    Xamarin.Essentials.Platform.Init(this, savedInstanceState);
    global::Xamarin.Forms.Forms.Init(this, savedInstanceState);
    LoadApplication(new App());
}

The first two lines assign resources for Tabbar and Toolbar. We then call the base method, followed by the mandatory initialization of Xamarin.Essentials and Xamarin.Forms. Finally, we have the call to load the Xamarin.Forms application that we defined in the .NET Standard library.

You don't need to understand these files in detail; just remember that they are important for the initialization of our app.

Xamarin.Essentials is a library that offers a lot of cross-platform APIs to commonly used resources on your phone, such as the accelerometer or the compass. Check it out at https://docs.microsoft.com/en-us/xamarin/essentials/.