Per Database Encryption Key in a MongoDB Cluster when enabling customer-managed keys with azure key vault

This Mongodoc covers managing customer keys with the azure key vault. There is a section that says MongoDB Server maintains a per database encryption key in the MongoDB cluster. Where are these keys stored and how are they accessed?

My scenario is this: I have a cluster that contains 100 databases. I have a need to delete one of those databases, including any backups of that database, but leave the remaining 99 databases intact.

If I had access to the encryption key for that database, I could delete it, thus rendering any backups for that database useless.