What is the main idea in a paragraph?
See explanation below
I think, it depends on what you are writing about as well. We use paragraphs to make clear what we are writing about.
But you don't want to talk about different concepts in a paragraph.
In a paragraph you focus on 1 concept only and finish it in that paragraph.
E.g. if you are writing about 1 specific character, you don't write about other characters in that paragraph, you start a new paragraph.
But if it is a conclusion paragraph, it summarises everything what you've said, so it includes different things .