💡 You can make either the index finger, the middle finger, or any point in between, the middle of your hand depending on how you position your thumb.
✅ Since young player tend to have small hands, they should spread their thumb out wide (I'd also encourage older players to do the same. I mean, why wouldn't you want to control the ball better?). When you do this the index finger becomes the middle of your hand (for most people).
❌ If your thumb is narrow, you won't control the ball as well. That's biggest detriment. There are a bunch of good shooters that release off the middle finger with a narrow hand, but remember that most of these guys are way bigger than the average person. Steph is "short" at 6'3".
💡 So before you go and list the players that have narrow hands and release off the middle finger to justify why you do it, remember this... wide hands control the ball better, and when your hand is wide, the index finger is the middle of the hand.