Since 2020, App Engine apps are served at [PROJECT].[REGION].r.appspot.com.
Once your app is running in App Engine, you can use the following URL to send HTTP requests to the app: https://PROJECT_ID.REGION_ID.r.appspot.com where PROJECT_ID is the ID of the Google Cloud project that contains the app.
I think this is part of GCP moving internal DNS from global to regional.
The same reference says:
The REGION_ID is an abbreviated code that Google assigns based on the region you select when you create your app. The code does not correspond to a country or province, even though some region IDs may appear similar to commonly used country and province codes. For apps created after February 2020, REGION_ID.r is included in App Engine URLs. For existing apps created before this date, the region ID is optional in the URL.
Empirically, I can see that [REGION] resolves to 'ts' for an app deployed in australia-southeast1, but 'ey' for an app in europe-west3.
How can I derive what URL my app is going to deploy too in any region?
You can use the gcloud CLI. Execute the command
gcloud app describe --project=<PROJECT_ID>One of the attributes returned is
defaultHostnameand it will have a value with format[PROJECT].[REGION].r.appspot.com