MFS is can be found at /usr/src/servers/mfs. It has a main loop that waits for a message to arrive. When a message arrives, its type is extracted and used as an index into a table containing pointers to the procedures within the file system that handles all the types. Then appropriate procedure is called and it does it's work and return a status value. MFS then sends back a reply to the caller and goes back to the top o the loop to wait for the next message.
Refer to the File System chapter's Section 5.7. Implementation of the MINIX 3 File System to get a detailed view on the working of each funtion and program. There are a few variations in 3.2 version which can be easily unserstood by refering to the source code