According to Mongo, this (i.e., MongoDB\Driver\Manager) is an "entry point" for the extension:
"This class serves as an entry point for the MongoDB PHP Library. It is the preferred class for connecting to a MongoDB server or cluster of servers and acts as a gateway for accessing individual databases and collections. MongoDB\Client is analogous to the driver’s MongoDB\Driver\Manager class, which it composes."
copied from here: https://docs.mongodb.com/php-library/master/reference/class/MongoDBClient/
However, any comparison of the "mongodb" docs here on php.net versus the "mongodb driver" docs on mongo's site shows dramatic and ever-changing differences.