They are using CitusDB but it's still RDBMS and support most PostgreSQL SQL query without the need to rewrite your query or change your schema.
I dont know if they are still using Vitess or migrated to Google other relational Database "Spanner"
In general I've noticed that people outside Google assume that Spanner is much more popular than it really is.
if you work at google and its one of the option to choose from! Is there any reason to "not" choose spanner for new project and instead use something else?