A typical modern handgun will have a hammer, safety, magazine release, and trigger. (edit: no, a modern handgun will have a firing pin instead of a hammer) There’s a pattern for correct usage that’s easily repeatable and committed to memory. It’s great for fidgeting.
The problem is that using gun as a fidget toy will totally fuck up discipline for all four of the fundamental firearm safety rules (for when it’s not a toy):
Treat all guns as if they are always loaded.
Never let the muzzle point at anything that you are not willing to destroy.
Keep your finger off the trigger until your sights are on target and you have made the decision to shoot.