Start with 2D mouse aim tutorials. There are many and there are many subtleties related to how they all function, so you would need to do the tutorial in order to express exactly what features you want.
If you don’t know, or you’re not willing to participate, perhaps you shouldn’t post? To be honest, and I’m being completely honest, I think you’re harming this forum with your spamming. I’ve seen you responding to my questions here, and you either copy and paste a small book linked to your own content, or make a comment that turns out to be programmatically false.
I would set up 2D IK first, then it shouldn’t be too hard using some math to figure out where the position of the gun should be. I believe this Tarodev video should be sufficient for the IK.
For the hand position, should just be Input.mousePosition then you turn that into Camera.ScreenToWorldPosition and then you subtract from the character’s neck position, and then normalize it. Something along those lines, the math might be a little jumbled though.