From what I’ve seen most of these people use the published numbers. The dev from AH who did the flight model used an extensive set of published numbers that were recorded when the real aircraft it was going through initial flight testing. These are very comprehensive reports, regarding aircraft behaviour at different trims, power, and rpm, etc…
For most freeware modders, I believe they use whatever is published in the POH.
And yeah, Asobo are shallow. Many of the default planes don’t even match what is in the POH.