If you rearrange someone else's ideas and use different words and sentence structure, have you really plagiarized?

Unless you at least mention the persons name in the idea then no but if your haven't then yes this would count as plagiarizing.

Hope this helped you out.

I think it depends on what the subject is. If for example, your subject is a debate where you can either be on one side or the other (therefore there are only two options), and you are either indifferent to either side, so you instead take up the position of someone you know or online, just for the sake of the debate, then personally, I do not think it is plagiarism.

However, if you are writing an essay on a specific subject and you look up the prompt online, see that someone else has written an essay on the same prompt, so you take their main ideas, simply putting it into your own wording to write your own essay, this is what I would definitely call plagiarism.