If I recall correctly they used some exotic database that is made for multi tenant cloud environments and scales up really well, but does not scale down too much. I've seen some article where a guy managed to set it up locally but it was a long one, and the result was resource hungry. But, again, my knowledge may be outdated

Just look at the setup docs. First they tell you to setup mysql and then this nugget

The correct way to authenticate with Spanner is by generating an OAuth token and pointing your local application server to the token. In order for this to work, your Google Cloud account must have the correct permissions; contact the Ops team to ensure the correct permissions are added to your account.