You’re asking for stuff kinda down in the architectural weeds a bit, so it’s not too surprising it’s hard to find. This stuff has grown a lot over the years. The forums are particularly difficult to search.
Best I can suggest is a google search ala “site: weatherflow.com data buckets” and start reading. The link @vreihen’s provided is the first hit there.
Another link might be Downloading all the back data - but be a little careful about believing ancient references to test API keys etc. as that has changed over the (five) years since that discussion. I would doubt how WF stores and aggregates (or not) data has changed much, in the absence of info saying otherwise.
I think that in the original air+sky days some folks did tests indicating that the bucketed info was the average info over the time period specified but I’ll never be able to find the old threads from several years ago. It might be easier at this point to just do the test.
Grab the readings over a time period from the UDP. Average them up. Grab the same time period via the WF server interfaces. Compare.