This question came up in a combat range discussion not long ago. Indeed, when does the aiming process begin? Most present said something about aiming beginning when the gun is at the full extension of the arm and the front sight is visible. Possibly, but I think it happens earlier in the sequence. As a matter of fact, I can make a pretty good argument for aiming beginning at the moment you realize you need to shoot.
In this sense, aiming begins wherever your gun might be on your person and a threat presents itself. It can include the draw if the gun is holstered, but either way, you must establish a positive firing grip. Without a proper grip, which aligns the handgun correctly in the hand, you cannot get a proper view of the sight system.
Hopefully, you will get level ground without obstacles around wherever the fight starts on which to get your stance, but hopefully I will also start getting younger any day now (not going to happen). The point is that good training will create good tactical and gunhandling skills that become habit. And they are part of the procedure that happens in a flash under stress and leads to the quick sight picture—the aim—that saves your bacon. This only comes with training.