I have a feeling that when what we are talking about is a general idea, we use the present perfect to refer to the past event instead of using simple past.
Is that the way to look at it?
Is that a difference between British English and American English?
(1) When people make friends with other people, people try to understand what others have said / what others said.
I think "what others have said" is more idiomatic than "what others said."
(2) Students in class often listen to the teacher and write down what the teacher has said / what the teacher said.
I think "what the teacher has said" is more idiomatic than "what the teacher said."
(3) When people want to learn cultures of different countries, they have to pay attention to what has happened in the country / what happened in the country.
However, in this case, I think simple past is better.