What is the setting of Nikon D90 and M files? Are there any related tutorials? I am a novice.

M file is a full manual exposure mode, which should be set according to your own shooting needs. For example, if you want to solidify an object moving at high speed (such as a clear picture of a bullet penetrating an apple), you must use a shutter speed of at least 1/4000 seconds. If you want to represent sports (such as making streams look like satin) or night scenes well, you have to shoot with a tripod at a very slow shutter speed (at least less than one second). Exposure (combination of shutter and aperture) can generally be adjusted according to the metering instructions in the machine (in special cases, exposure can be increased or decreased by itself). Therefore, M file gives photographers the greatest creative freedom, and it is also the basic skill that photographers must master.