Wrist watches usually use the barometric pressure sensors they have to calculate the altitude. I have also confirmed this from the manual of my watch, which states that:
The watch displays altitude values based on air pressure readings taken by a built-in pressure sensor.
Fine that's understood and makes sense and has always worked for me wherever I go.
But then how does it know altitude during flight? The cabin of all commercial airliners are pressurized and hence the air pressure is not changing inside the cabin with the change in altitude (as happens for descent or ascent on ground). I know that even the aircraft itself uses pressure sensors for many calculations but those sensors are mostly usually on the Pitot tube and they sense the real outside pressure and not inside an pressurized cabin
I would also like to mention that yes the altitude reading it takes during a flight is not always close to what the real value is but still it does track the altitude difference when pressure inside the cabin remains same, how so?
That I didn't include the exact details of my watch because neither am I promoting any special model or a brand nor am I asking specifics of how a watch works, focus is on air pressure inside the cabin, not on watch. If needed I can provide the model and manual reference links.