In simple words, a general Database Engine is a big clunky piece of software with features for all the use cases, and its up to you to choose which features to use. Whereas in a purpose built database, you get a lean, specific database which is only suitable for the feature you want.
For instance, AWS offers 15 purpose-built database engines including relational, key-value, document, in-memory, graph, time series, and ledger databases. GCP also provides multiple databases types like Spanner, BigQuery etc.
But the thing is that the one-size-fits-all monolithic databases aren't going anywhere. They are here to stay. A medium to large organization has way too many requirements and features to be used and having one database for every use case increases the footprint and cost. For every production database, there is a dev, test, and QA database so the foot print keeps increasing.
So the thing is that though having purpose built database notion is great it's not going to throw monilithic database out of the window. It just provides another option for the organization and they could just have a managed service for purpose built database for a specialized use case but for a general database requirement for OLTP and data warehouse, monilithic is still the way.