Turns out the project is using two different wrapper libraries (?!):

1. Firebase JS SDK firebase for authentication, which uses config.js to reference environment variables. https://docs.expo.dev/guides/using-firebase/
2. react-native-firebase for analytics, which uses the google-services.json file https://rnfirebase.io/#installation-1

While I'm here, I also learned that OAuth "Sign in with Google" requires the Web app id, not the Android/iOS app ids.