How do you aim the weapons in MGS1 and MGS2 without firing them? On PS1 and PS2 you just held down the square button lightly to aim, then pressed it in further to shoot.

Is it possible on these PC ports?

MGS1 didn't have this feature because there were no analogue buttons on PS1 and MGS2 on PC does not support analogue buttons or the PS2 controller. With Direct Input controllers you can aim, press X (or Square or left button) to get your pistol ready for firing and then hold L3 (left stick button down) and you will lower your weapon without firing however this only works on certain Dinput devices such as Nintendo Switch Pro Controller when connected via bluetooth so it's very hit and miss on the type of controller you have. There is a better solution especially if you are using an Xbox controller which is to use the MGS2 V's Fix you can find on Github (be sure to follow the instructions) that will bring back the L3 ability on the Xbox and PS4 controllers natively.