Hi Austin,
Nexus 3 has used an embedded OrientDB database, with 3.31 we announced initial support with temporarily limited format support for migrating to PostgreSQL or H2 (another embedded database, albeit a more traditional relational one). The migration is less time consuming than the 2->3 migration as it only moves database contents.
Pro users switching to PostgreSQL would need to manage the PostgreSQL instance outside of Nexus, this can of course be a managed service like Aurora if running in AWS. With H2 the deployment is similar to today where the database is simply part of the Nexus service and needs no external management. While H2 is fast (as communication is in-process rather than network) we anticipate that with large volumes for data PostgreSQL will be preferable, and that PostgreSQL enables more resilient deployments, backup options, etc…
If you’re looking for more information we have a section in our help documentation.