Apologies about not searching prior to posting - but I'm not even sure what to even search for, and I've hit the ground running on a graphics job (I'm an experienced video producer/editor without Illustrator experience). So here goes:
I'm using one of the .ai templates as a starting point. There's a mask with 4 rounded corners. I've placed new graphics in various positions on top of this mask, but cannot figure out a way to get any of the squared corners of the the new graphics to fit a matching underlying rounded corner.