I use one finger when typing "loop", because it doesn't make sense to do otherwise given how the respective keys are on QWERTY. That just means the word "loop" sucks.
Regarding parentheses/brackets, I said "separate fingers", not "separate hands". I do use separate fingers for parentheses/brackets.
The numpad is a weird thing. When I've (rarely) needed it for sustained number capturing, I do typically stick to one finger because it's easy to get lost with multiple. Not sure why. It could just be lack of practice with the numpad.
For me, the "P" key is right at the split between zones. I probably (nearly?) always type "O" with my middle finger. But, the "P" is easily done with middle and ring fingers, and it seems like I may split the duty in many words.
It's hard to be sure, because I'm not sure how much I bias my movements by trying to monitor them while I do it! Normally I just type on auto-pilot, looking at the screen and not really being aware of the specific movements or finger duties...
I have a very idiosyncratic touch-typing method. I am self-taught with 40+ years of computer usage since grade school. There are arcing zones that are in the natural reach of different fingers, but with overlap where adjacent fingers can reach the same keys. I think my middle fingers are somewhat dominant, and may do more than their share of the total keypresses because of their dexterity and reach.
I have relatively large and flexible hands, so I span the keyboard by bending and reaching fingers rather than moving my forearms. The outer heel of my hands are statically parked on the palm rest or desk, and just the small joints in my hands and fingers are moving.
This parked posture is probably why I strongly prefer the Thinkpad "trackpoint" input device for general computer use.
Regarding parentheses/brackets, I said "separate fingers", not "separate hands". I do use separate fingers for parentheses/brackets.
The numpad is a weird thing. When I've (rarely) needed it for sustained number capturing, I do typically stick to one finger because it's easy to get lost with multiple. Not sure why. It could just be lack of practice with the numpad.