Durability of storage in second generation google cloud sql How durable should I expect data for a Google Cloud SQL instance to be? Just a single instance, with the high-availability option disabled.

AWS explicitly mention that Amazon Aurora data is stored in six copies over multiple availability zones. Is any such information available for Google Cloud SQL?
You may not find any such information for GCP because Aurora is a storage engine native to Amazon. It can be used with MySQL or PostgreSQL.
You only have three(3) options here:
OPTION #1
You need to read the Amazon Documentation on Aurora and try to locate the same storage options and features in CloudSQL. Thoroughly scour the GCP Documentation for any new storage options for CloudSQL similar to Aurora.
OPTION #2
You should also focus on what Google provides for CloudSQL High Availability

Overview of the High Availability Configuration
Configuring an Instance for High Availability
Instance Locations

OPTION #3
You may want to consider just customizing a solution with ComputeEngine instances with MySQL manually installed and create frequent snapshots of your data folder (which should contain InnoDB tables only along with binary logs). Additional instances (at least 1) for setting up MySQL Replication slaves may be need to still have additional HA.