herefore, if I set the altimeter to 0 meters, the hPa will magically and automatically change to the "correct" value of.....?
Pressure readout always is (or should be) the current pressure.
The reason to calibrate is to tell the altimeter the current pressure altitude.
If you are sealevel, and you calibrate (which you can do in 2 ways in the settings: either set the altitude to 0, OR set the current pressure), the watch will (should) show you at an altitude of 0.
If subsequently the pressure changes, you will see the pressure change in the barometer, but you will also see a change in the altimeter (even if you don't change altitude). The reason is that the altimeter just follows the rule that "pressure changes approximately 1 hPa with every 8m", so if pressure drops with 1 hPa, it will show you an altitude of 8m. If you are indeed moving up and pressure is constant, this will give you a good estimate of your altitude. If however it is just a pressure change and you stay put, you should recalibrate.
The reason for the 2 ways of calibration is that it may be that you know your altitude (e.g. some sign on a hiking path) or you may know the pressure at sealevel. This is what the pressure WOULD BE at your location IF you would be at sea level; this follows the same 1 hPa/8m rule; airports supply this to allow pilots to calibrate their altimeters (it is called QNH). But you always need to know one of the two (your altitude or the QNH) in order to calibrate; autocalibration takes your location and uses that knowledge (I don't know if it uses gps altitude or looks up the regional QNH from your position).
You can check that if you change the calibrated altitude, it also changes the calibrated pressure and vice versa.
