CrateDB is QuantumLeap's default backend where NGSI data will be persisted. In addition to using QL's API, if you want to by-pass QL, you can also interact directly with CrateDB to query all the data QuantumLeap has stored from the received notifications. This of course is not recommended as your implementation will depend on QL implementation details that may change in future.
If you followed the Installation Guide, you have a ready-to-use CrateDB instance running in a Docker container. The easiest way to interact with it is using its admin interface, as documented here. Alternatively, you can use its HTTP api, or any of its supported clients.
You can learn more about CrateDB by reading the docs.