Often you’ll need to copy a layer mask from one layer to another so you can mask out the same area on two layers. I do this when I mask the
hightlights highlights in an image to protect them from being blown out.
Copying a layer mask doesn’t look easy or intuitive – there’s no menu command for it. However it can be done very simply. Hold the Alt and Control keys at the same time and drag the layer mask from one layer and drop it onto another. If there is already a layer mask on that second layer you’ll be prompted to replace it, answer Yes to the prompt.