what is binary in binary json (BSON)?
It's an analogy semantics for binary data types. BSON binaries has a array of bytes representing a internal subtype from 0 to 127.
For example: the byte 0000 0000 1000 represents the hexa "\x08" which means the type boolean in BSON specificiation.
The driver will be in charge encoding and decoding formats UTF-8 to BSON and Storage Engines such as WiredTiger in MongoDB will responsible for serializing and deserializing at data in BSON format.