Google Cloud SQL. I say magic because locally I need a service key to connect to the proxy, but the production app doesn't seem to need anything but the internal google address.
The service credentials are supplied via an env variable that points to their location. Locally, you can provide the location directly or set the env variable yourself. When deployed, most GCP service environments just have that variable setup already and you don't have to think about it, so it feels a bit like magic. Same thing underneath the good though.
What product is this?