'Save Layer as Transparent PNG' command can be saved the current layer as transparent PNG. You also can use this function if you want to save only the line drawing.
Prepare an illustration you would like to save a layer as a transparent PNG.
Open ① the Layer window, select ② the layer with the line drawing, tap ③ '…' button, select ④ "Save Layer as Transparent PNG".
The line drawing is saved with a transparent background in your photo library.