The story of YSFlight is long, but FLYBY2[0] screensaver for Windows 98 is probably one of initial implementations of YSFlight flight engine (aircraft 3D models distributed with FLYBY2 are now distributed with YSFlight in same DNM-format).
Few weeks ago Soji Yamakawa open-sourced[0,1] FLYBY2 and ported it also for FM TOWNS.
There is large list of addons created by users, which highly extends YSFlight.[1]
Also, there are a lot of open-source utilities for addons makers.[2]
Read «YSFlight Handbook»[3] and «YSFlight Scientific Research»[4] papers for understand addons formats and some internals of YSFlight.
[0] https://forum.ysfhq.com
[1] https://docs.google.com/spreadsheets/d/1caVHoWU6g1YSB-G-W5Q-...
[2] https://github.com/YSFlight-opensource
[3] https://forum.ysfhq.com/viewtopic.php?t=8172&p=92286#p92286
[4] https://sites.google.com/prod/site/ysdecaff/ysflight-scienti...