Question

I hear that the Android’s desktop is an application that I can change.
I’m searching for some information about how to do that.

Answer 1

I guess you mean when you press the home button you want to have your app there instead of the default home screen? And do you mean on a non-rooted phone?

I use an intent filter for that, i.e. I want my activity to start on phone boot-up and be the default home screen.

<intent-filter>
    <action android:name="android.intent.action.MAIN"/>
    <category android:name="android.intent.category.LAUNCHER"/>
    <category android:name="android.intent.category.HOME"/>
    <category android:name="android.intent.category.DEFAULT"/>
</intent-filter>

On a regular phone, the user will get an option to choose between your app and the default home screen though at first, although he can mark a checkbox then to always choose this app as the default screen then – but your app cannot force the user to do that.