Generally speaking “you” do not calibrate a measurement mic. I’m restricting the discussion to measurement mics because that’s pertinent to the OP and because mics have all sorts of frequency responses designed into them on purpose.

Measurement mics (the professionally targeted kind) can with some models be purchased from the manufacturer with a calibration file that can be used by various audio measurement software packages. There are also places that will calibrate your mics for you. In the end what this really means is just figuring out the frequency response characteristics of a given mic relative to a known source. This could just be a case of “we match it to this mic over here.” As long as all the mics someone is using are calibrated to the same standard then it’s less important that they’re perfectly flat in frequency response. I have some really good mics but the bulk of my measurement mics are inexpensive and not calibrated. They work fine though because they all measure close enough to the same that once you figure in all the external parts of a system (the room for instance) the mics minor variations become insignificant. The important thing is that I can put 8 mics around a room and pull reliable data to tune the room and make it sound musical. At the end of the day all the measurement stuff is just there to help you scientifically quantify why a room doesn’t sound right and then you can fix it. Like the last room I tuned that had 2 blown HF drivers and the polarity flipped on some LF drivers. The ears easily said there was a problem. The tools told me right away what they were. Sure I could have found them without the tools and by tweaking settings till everything was right, but the proper tools (and knowing how to use them) makes a huge difference.

That's a great question! I have no idea. At low frequencies it should be very easy, because the sound is just a pressure measurement, so you can compare against a calibrated pressure reference. So the main challenge is measuring the high-frequency amplitude and phase response. If I had to do this, I'd probably set up a speaker in a long box with standing-wave resonance modes, and put both the microphone-under-test and a laser interferometer at an antinode to measure the change in refractive index that occurs with air pressure. A photodiode should have a flat frequency response out to well beyond 20 kHz, so that would do well as a calibration. But this is probably overkill for audible frequencies.

Generally you look up your microphone's magnitude response and enter it into whatever interface you have. For home audio receivers they have this hard coded for their included microphone.

Also, generally microphones are sensitive to orientation, so to have repeatable results be sure to control that. Position in a room is also essential for repeatability.