Hello @Kim_Hakseon,
By default, MongoDB’s storage engine is WiredTiger. But, MongoDB server can be configured with other storage engines. The way the files are created and stored is different for different storage engines. In the MongoDB server versions prior to v4.2, there was an option to use MMAPv1 storage engine (see Storage Engines v4.0).
The mongod can be started by specifying the –storageEngine option. In case you have started the server using a specific storage engine, and then try to restart with another storage engine, the server will not start - when the dbpath points to the same directory as before (the --dbpath option is used to specify the directory where the database files are stored - your collections, indexes, definitions, data, etc.).
NOTE: Starting in version 4.2, MongoDB removed the deprecated MMAPv1 storage engine.