ソースコード設定

下記適切なバージョンを使用
https://developers.google.com/admob/android/quick-start?hl=ja

build.gradle

dependencies {
    implementation 'com.google.android.gms:play-services-ads:19.3.0'
}

ネットワーク権限などを設定

AndroidManifest.xml

        <meta-data
            android:name="com.google.android.gms.ads.APPLICATION_ID"
            android:value="ca-app-pub-3940256099942544~3347511713"/>

レイアウト記入

activity_main.xml

    <com.google.android.gms.ads.AdView
        android:id="@+id/adView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_alignParentBottom="true"
        ads:adSize="BANNER"
        ads:adUnitId="ca-app-pub-3940256099942544/6300978111">
    </com.google.android.gms.ads.AdView>

Activityに記入

MainActivity

override fun onCreate(savedInstanceState: Bundle?) {
        // Test App ID
        MobileAds.initialize(activity, "ca-app-pub-3940256099942544~3347511713")

        val adRequest: AdRequest = AdRequest.Builder().build()
        adView.loadAd(adRequest)

        // ad's lifecycle: loading, opening, closing, and so on
        adView.adListener = object:AdListener(){
            override fun onAdLoaded() {
                Log.d("debug","Code to be executed when an ad finishes loading.");
            }

            override fun onAdFailedToLoad(errorCode : Int) {
                Log.d("debug","Code to be executed when an ad request fails.");
            }

            override fun onAdOpened() {
                Log.d("debug","Code to be executed when an ad opens an overlay that covers the screen.");
            }

            override fun onAdLeftApplication() {
                Log.d("debug","Code to be executed when the user has left the app.");
            }

            override fun onAdClosed() {
                Log.d("debug","Code to be executed when when the user is about to return to the app after tapping on an ad.");
            }
        }
}