The full answer
PostgreSQL is relational: tables, foreign keys, constraints and ACID transactions. That makes it the right default for payments, bookings, inventory, HR, ERP and anything where an incorrect or half-written record causes real problems. Its JSONB columns also handle semi-structured data well, which covers many cases people assume need a document database.
PostgreSQL's extensions widen its reach. pgvector adds AI similarity search, PostGIS adds geographic queries and full-text search is built in, so one well-managed database often replaces several specialist systems.
MongoDB stores documents whose fields can differ from record to record. It suits product catalogues with varied attributes, user-generated content, IoT telemetry and logs, and it scales horizontally for heavy write loads with simpler sharding.
Some products use both: PostgreSQL for the transactional core and MongoDB for high-volume or loosely structured data, with Redis for caching and queues in front of either.
Whichever is chosen, Creuto designs the schema and indexes early, uses managed database services for backups and failover, and encrypts data at rest. Switching databases later is expensive, so the reasoning is written into the proposal for you to review.