I personally use colored pencils for this, but I'm the only one who does that.. It's just pointillism, most people use sharpies or markers. In darker parts, you put the dots closer together, lighter parts they're further apart. That's how you create the contrast.
Thank you! I'm always a bit reserved before starting a new pointillism, because each one consists of thousands of dots.. Just the part on the bottom right is over a thousand dots already, I counted that bit when making it! And because I use coloured pencils, it means a lot of tiny wrist movements, which hurts when I do it for too long in one sitting!