I have left exactly one review in Google in my entire life, and it was a 1-star review of a car rental place that royally screwed up.

A couple months later I was notified that "x users found this review helpful". So someone must have seen it despite me never reviewing anything else.

Those interactions from other users can be totally fake to boost engagement.

Possible, but it could also be that the reviewing system considers them a high-signal user: very few reviews most/all negative would be a person who doesn’t care for reviewing but had such horrendous experience they were pushed to warn others.

I operate like that, but the other way. I generally only leave very positive reviews when I've had a great experience. I guess I'm fortunate that I've yet to have an experience so bad that I was motivated to leave a 1 star review despite my apathy.