The people who got snapped turned to ash, no bodies left behind.
If you know a specific person/entity caused it and what the reason was (even if it is a stupid reason), I think your response to the event is different than if (ala The Leftovers) some percentage of all the people on your planet suddenly disappeared.
It’s called “ash”, but it’s not - these people didn’t burn, they were disintegrated. And I don’t know if there was any explanation which said this biomatter is not digestible by bacteria - if there is, I’d like to see it. (Serious query, not snark.)
Exactly. There are a few responses from societies that never heard of Thanos (or Infinity Stones)
A scientifically advanced society would investigate. They might figure out what happened.
A superstitious culture probably would turn inward and embrace religion.
A super advanced civilization might both figure out what happened and then find a way to do something about it. So some alien from 100 million light years away might undo what Thanos has done. Or get revenge by killing everyone in this galaxy.