Essentials uses the Apache Derby Database when creating a backup on File System Storage. Its an open source relational database implemented entirely in Java. No extra tools or services are required to be installed. (https://db.apache.org/derby/)
For Azure Storage, Essentials uses the Table Storage Service - a NoSQL key-value store for rapid development using massive semi-structured datasets. In order for this to work, you must have the Table Storage Service. (https://docs.microsoft.com/en-us/azure/storage/tables/table-storage-quickstart-portal)
For Amazon S3 storage, Essentials requires you to have the Amazon DynamoDB service. Amazon DynamoDB is a key-value and document database that delivers single-digit millisecond performance at any scale.
If you use Amazon S3 storage with Custom End points in Essentials with Amazon DynamoDBLocal service, then you do not require an amazon DynamoDB. This is because the DynamoDB databases are placed locally on the file system and synchronize with backup project located on Amazon S3 storage. (https://docs.aws.amazon.com/en_us/amazondynamodb/latest/developerguide/DynamoDBLocal.html)