Multiple masks with subtractions

I wanted to create a masking that would exclude the sky and also the bottom 10% of the image. So I created a Sky mask, then I inverted it, then I subtracted from the range a linear gradient of the bottom of the image.

I was completely confused. Somehow my modifications to the image were not restricted to the combination of the two masks but sometimes to one or the other. At one point I was modifying only the bottom part of the image which I had thought was explicitly subtracted out.

I couldn't understand the iconography in the bottom right pull-out showing the mask layers, and because this is the Android version (I guess) there were no tool-tips to help.

Can someone help me get on track here?

Also I couldn't find any way to do curves within a mask, only the regular light editing sliders. Are curves not combinable with masks?

FYI I'm using the Android app on a Chromebook.

Thank you!

-- Ethan