Airpressure vs height

So simple question.
how does the watch know whether the airpressure is dropping/increasing or that I am actually changing my height?

It doesn't.

Every time I correct my actuel height a few hours later the actual air pressure has changed but the fenix thinks I have increased or deceased in height while I just been a sleep.

This is normal and there is nothing you can do about it.

If you go to the Barometer setting and the "Watch Mode" setting is set to "Auto", the watch should lock elevation if you are not moving (linked to the activity tracker registering steps) or assumes barometric pressure (adjusted to MSL) is constant whilst you are moving. So when you are asleep, your elevation should not change because you are not moving (there are settings on the Fenix 5 Plus series that means this is not necessarily the case, as the watch does try to calibrate overnight based on data from your phone).

Intended behaviour for elevation vs weather changes to pressure explained here https://support.garmin.com/en-GB/?faq=J2WVA0dss82BNpphPYWq56&productID=603267&searchQuery=auto%20cal%20not%20during%20activity

Watchmode is set to auto but still overnight the altitude is changing a little most of the times it's like 20 meters while it must be 7.

You could try changing the Watchmode to “Baromter” overnight, which should lock the altitude and apportion all pressure change to weather. https://www8.garmin.com/manuals/webhelp/fenix5x/EN-GB/GUID-CDC9DDEE-C576-4E1D-B007-BC737686793A.html

A barometric attitude sensor will never be accurate over time. This is why pilots set the pressure to that of the local airport whenever going below 18.000 feet on approach - and set it to 1013 mb above, to have the same reference for all flights when well above the ground.

Pressure can vary a lot even within a few KM , the watch can only guess if you are moving up/down or pressure is changing. If pressure is changing while you are hiking in the hills you are absolutely out of luck ????

What Michael said! The whole process is far more complicated than some understand. An old but useful post here provides a good overview of ABC watches https://forums.watchuseek.com/f374/altitude-barometric-pressure-abc-watches-explained-266991.html (also a little added info on the last comment page 3). Different brands use different tricks to figure out what is actually changing and although some may be better than others, all of them will drift and have troubles at times.

Thanks.
There is an storm incoming in the Netherlands and the altitude has gone down from 6 meters to - 22
The above is making things clear, aspecially that it's just the way it is :)

Precisely - Storm coming in usually means a cold front/high pressure. Increased pressure, watch thinks you are at a lower altitude. Some watches look at the rate of change and regard slow changes as change in air pressure and fast changes as changes in altitude - but that may fail if you move very slowly or the storm comes fast.

I know some hikers calibrate their altimeter every time they pass a peak with a known height - several times per day.

Personally, I don't navigate on specific contour line, so I don't really care about the fluctuations.