Cloud SQL is PAAS (Plaform As A Service).
Cloud SQL is a docker container built on top of a GCE instance, and Google monitors everything for you, and fix the Cloud SQL instance automatically if something goes wrong (Sometimes Google software engineers have to perform some actions to fix some issues if the instance is stuck). So, the only thing that you have to take care of is to store and query your data.
Also, Cloud SQL offers a lot of interesting features, such as failover replicas, read replicas, user and database administration, etc.
So, in Cloud SQL, Google doesn't just sell the infrastructure to create databases, but also the application itself and the monitoring tools too.