What runs on it
Every part of the platform that needs a system of record, the pricing engine, the order lifecycle, quote requests, the organization hierarchy, the audit log, reads and writes to the same PostgreSQL database. There is no separate proprietary store for any of it.
Why a standard installation matters
Because it is stock PostgreSQL rather than a proprietary fork, standard backup tooling, monitoring, replication and hosting options all apply without modification. You can run it yourself, hand it to a partner, or use a managed PostgreSQL service; only the connection string changes.