You might be thinking of MTP, used by eg. phones that don't present a block device over USB so that they can internally use a filesystem not supported by Windows; MTP is a higher-level interface more akin to a network file system protocol.
mtd is a Linux driver subsystem for accessing raw NAND or NOR flash devices that don't have an SSD's Flash Translation Layer (FTL) in front of them to provide features like wear leveling. It's most commonly encountered in embedded systems.
As I understand it there is a controller on your device that emulates a disk drive. The controller is accessing the raw device on your behalf and _should_ not be overwriting any sensitive/forbidden areas of the flash that might brick the device.