How altitude changes with changed QNH?
Yes. There's an old saying: when flying from high to low, look out below! https://sabushpilot.com/high-to-low-look-out-below/ has nice properly descriptive image.


  Airplane is maintaining certain FL with some QNH


Note that if you were at a FL (flight level), you wouldn't be using QNH, you'd be using QNE. You use QNH up to the transition altitude and clearances will be given to maintain a certain number of feet (or meters in some countries). Above the transition altitude, your clearance would be to maintain a certain FL and you would be using QNE, 1013.2 hPa.