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?
They are not just some random 3 have decades of real db experience behind them. They also just got funded which gives them the ability to expand and stay longer in the game.