A database named "test" is created in mongodb 4.0

Question

{
    "set" : "rs0",
    "date" : ISODate("2022-03-02T02:22:19Z"),
    "myState" : 1,
    "members" : [
        {
            "_id" : 0,
            "name" : "link:port",
            "health" : 1,
            "state" : 1,
            "stateStr" : "PRIMARY",
            "self" : true,
            "uptime" : 6660018
        },
        {
            "_id" : 1,
            "name" : "link:port",
            "health" : 1,
            "state" : 2,
            "stateStr" : "SECONDARY",
            "uptime" : 6660018
        },
        {
            "_id" : 2,
            "name" : "link:port",
            "health" : 1,
            "state" : 2,
            "stateStr" : "SECONDARY",
            "uptime" : 6660018
        }
    ],
    "ok" : 1,
    "operationTime" : Timestamp(1646187739, 1)
}

Database and collection configuration

The environment that is experiencing the problem now consists of a database, a replica set, and a shard as above.

There were no human errors in the backend App. and, The source code for creating a database named test did not exist. But nevertheless, a database named "test" is still created automatically.

I wonder why such a database is created if it is a function for the operation of sharding or replica set.