I was going to say left but I just checked and it's my right .. like almost exclusively .. for letters on both my left and right hands. My caps lock is mapped to control and I use my left pinky a lot for that...
I normally use the opposite hand. That is, if the letter is typed with the left hand, then I use the right shift key. If typed with the right hand, then I use left shift.
I always use my left hand. My left hand is quite dominant on my keyboard. There are only a couple keys that I use my right hand for. fwiw I can type reasonably fast.
Opposite hand. Learned this at a 10-finger method typing course primary school offered at the time (on a typewriter).