Copper1 is the top layer, copper0 is the bottom layer. On a single sided board you only need copper1, but it becomes the bottom layer (copper0). When you have a 2 sided board and switch it to a singled sided board, copper1 then becomes copper0, (the bottom layer). Through holes use both copper1 and copper0, but a solder pad can be put on top (copper1) or the bottom (copper0),
So far, I haven’t figured out a way to put a solder mask over a solder pad, I think it would need to be on the trace layer. If I figure out a way I will let you know…