Hello! I'd like to learn how to get the look of these icons:

icons by the lovely lucific
I want to know how to get the circles that are there. I know it's the pen tool, but I don't know what settings. I'd use Pencil or Paintbrush as the setting, and it would be too pixelly or blurry.
Help please!
EDIT: I've found out how to do it. Thanks to everyone who helped!