I see you've never had a child who knew what was coming with an immunisation, and was determined to do whatever it took to avoid being stabbed with a needle, then. One of mine was fine, but the other resisted as a toddler and child and tried to fight no matter what you did. There was no 'keeping them calm' through careful planning or comfort - it took the strength of their large dad holding them pinned on his lap in order to do it safely.
As for car seats, again, mine were mostly fine, but at two to three, every so often they would fight being put into the car seat. You'd pop them in, they'd fight it, you'd give them a minute to calm down or even take them out and let them run around for a moment, you'd try to bribe them, or reason with them - but when you put them back in, they went straight back to being as stiff as a board and flailing. We could do that for up to an hour without them acquiescing - every single time they went back in the seat, they'd fight. It just wasn't tenable.
However, if you did just push them back by the abdomen, or by their nappy/crotch when they weren't yet potty trained, then as soon as the clip clicked into place they'd accept it and stop fighting. Never any injuries, marks, or bruises! It didn't even hurt them. It was just a very short burst of strength to push their bums back enough to click the clip into place.
I think possibly you have little experience of very difficult, incredibly stubborn children who are utterly relentless.