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Activity :-

MainActivity is an example of an Activity. An Activity is a core Android class that draws an Android app user interface (UI) and receives input events. When your app launches, it launches the activity specified in the AndroidManifest.xml file.

Each activity has an associated layout file. The activity and the layout are connected by a process known as layout inflation.

AppCompatActivity :-

AppCompatActivity is a subclass of Activity that supports all modern Android features while providing backward compatibility with older versions of Android. To make your app available to the largest number of devices and users possible, always use AppCompatActivity.

R Class :-

R class is generated when you build your app. The R class includes all the app's assets, including the contents of the res directory.