how to get android app_id from google play services

For the redesigned google play console (2020), click the Play Games Services, then click setup and management, then click Configuration. You will see information about your app, one field which is Authorization, and there you will find the application ID.

Sign in to the google developers console here: https://play.google.com/apps/publish

You should be able to find it under your app.

EDIT: It's in the Games Services tab