I just got done implementing my own interface in C++ with Polygon’s api, so I understand the frustration. I’ll take to time to answer your questions in what I hope you find satisfactory.
I know of two aggregate timespans: minute and day. When the multiple is 1 and the timespan is minute, you’ll get bars that span one minute of time. When the multiple is 5 you’ll get 5 minute bars, and so on.
It’s a super simple api and really that’s all you need to know! Now if they’d just put that in their documentation…