If you want to put AdMob ads in your Android app, you will need to specify your AdMob app id in the Android manifest file. However, I am committing this file to source control, so I didn't want to hard code it in there. The Android manifest file is an .xml one and, as such, I had no idea how to even have variables in the first place. Luckily, I found this post, what follow is just a summary mostly for my future self on what to do for Android.

1.- In build.gradle, type the following somewhere above android::

def dartEnvironmentVariables = []

if (project.hasProperty('dart-defines')) {
    dartEnvironmentVariables = project.property('dart-defines')
        .split(',')
        .collectEntries { entry ->
            def pair = new String(entry.decodeBase64(), 'UTF-8').split('=')
            [(pair.first()): pair.last()]
        }
}

2.- In defaultConfig:, add the following:

manifestPlaceholders['APPLICATION_ID'] = dartEnvironmentVariables.APPLICATION_ID

3.- In AndroidManifest.xml, you can now use the variable. In the case of AdMob it needs to be added as meta-data as so:

<meta-data 
          android:name="com.google.android.gms.ads.APPLICATION_ID"
          android:value="${APPLICATION_ID}" />

4.- That's it, remember to either flutter run or flutter build with the env variables, otherwise your app won't build correctly:

flutter run --dart-define APPLICATION_ID=1234qwerty --dart-define WHATEVER_OTHER_VARIABLE=9876poiuy

and

flutter build appbundle --dart-define APPLICATION_ID=1234qwerty --dart-define WHATEVER_OTHER_VARIABLE=9876poiuy