Masking/Unmasking an object

Hi there!

It probably is simple, but I’m just getting started and can’t find this function!
How to you mask/unmask an object?

Thanks!!

Found it!

However, is there a way to reveal an object let’s say in a circular manner?
Not only by stretching/expanding this square shape around it?

Thanks!!

You can keep add radius to the mask until it become circular manner.

Thanks!!