From your AdMob account, create an app for iOS. You will get an app id for iOS.
Create 3 ad units. One banner, One Interstitial, and One Rewarded video.
Now, go to the ios/Runner/Info.plist file and change the AdMob app(iOS) with yours.
Go to the lib/configs/ad_config.dart file and change the appIdiOS, interstitialAdUnitIdiOS, rewardedVideoAdUnitIdiOS & bannerAdUnitIdiOS with your values.