The source code is available for inspection as is the biographies of the people involved.
They rely on the libraries that are part of Postgres itself to ensure they are parsing the SQL etc "correctly" (where "correct" means "the same as Postgres itself).
Bigger organizations do not necessarily mean higher quality.
What bigger organization is testing PostgreSQL itself?
What are the relative quality measurements of Postgres vs MariaDB vs Oracle vs SQL Server?
As long as they don't get undercut by the equivalent of AWS https://aws.amazon.com/rds/proxy/ which is a managed pgbouncer.