AdmobTest/Assets/GoogleMobileAds/Editor/GoogleMobileAdsDependencies.xml

<dependencies>
  <androidPackages>
    <androidPackage spec="com.google.android.gms:play-services-ads:21.0.0">
      <repositories>
        <repository>https://maven.google.com/</repository>
      </repositories>
    </androidPackage>
    <androidPackage spec="androidx.lifecycle:lifecycle-common-java8:2.4.1">
      <repositories>
        <repository>https://maven.google.com/</repository>
      </repositories>
    </androidPackage>
    <androidPackage spec="androidx.lifecycle:lifecycle-process:2.4.1">
      <repositories>
        <repository>https://maven.google.com/</repository>
      </repositories>
    </androidPackage>
  </androidPackages>

  <iosPods>
    <iosPod name="Google-Mobile-Ads-SDK" version="~> 9.0">
      <sources>
        <source>https://github.com/CocoaPods/Specs</source>
      </sources>
    </iosPod>
  </iosPods>
</dependencies>