MainActivity.java intro in android studio

A. MainActivity.java

package com.example.myfirstandroidapp;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

onCreate function is loading during open an android app.
setContentView set the xml file which is showing on app loading.