For one thing, they abandoned the idea of having a system detect the position and size of the whole hand. Instead, they only have the system find the palm, which is not only the most distinctive and reliably shaped part of the hand, but is square, to boot, meaning they didn’t have to worry about the system being able to handle tall rectangular images, short ones and so on.
Once the palm is recognized, of course, the fingers sprout out of one end of it and can be analyzed separately. A separate algorithm looks at the image and assigns 21 coordinates to it, roughly coordinating to knuckles and fingertips, including how far away they likely are (it can guess based on the size and angle of the palm, among other things).
techcrunch.com/2019/08/19/this-hand-tracking-algorithm-could-lead-to-sign-language-recognition
techcrunch.com/2019/08/19/this-hand-tracking-algorithm-could-lead-to-sign-language-recognition