You have to use the transparency "paint". I think default key is x. it's the blank one next to your two colors on the color palette.
In Clip Studio Paint, it's a little different: you ERASE to mask, and PAINT to unmask.
Note that you can turn most brushes into erasers by switching your active color to Transparent (default shortcut C, or click on the checkered bar below the two color boxes on the bottom of your tool bar)
Importing frames to clip paint for referencing is not as straightforward. See this tutorial.
2D camera folder in clip studio. so you can just draw one big canvas and move your 2d camera around it.
can immediately drag audio to canvas and not to the timeline./