What MTP for Mac actually does
Ok, let's dive in to details! So, how OS X get to know what and how deal with different devices and protocols? It needs some special plug-ins! They are located in Kernel of OS X and know how to work with this specific device and represent them to user through VFS (Virtual File System - layer of abstraction which unifies your experience with file system at all).
As you already understand, the main component of MTP for Mac is such plug-in or Kernel Extension or some times it called "driver". This driver resides in a Kernel and wait until some MTP device will be connected to you Mac. On connection it negotiate with device to determine its specifics and represent it to VFS. Now you will see your Android phone or tablet like usual disk in Finder and on the Desktop. From this moment you free to use any software you like, with your photos, mp3 or any other types of files on you device!