No, there are no design documents... it's a poorly documented giant of code
But the subfolders are easy enough:
audio
conf: for the xml config stuff
content: holds the mods by the Exult team
data: holds the files Exult needs and the two games need
desktop: files needed for linux desktop
docs: well, it should have the design document
files: how to access and use files
flic: to handle the animations in the fli/flic format (SI has those in the intro/outro)
gamemgr: how to handle the different games and mods
gumps: is all about the onscreen gui elements
headers: headers we need and include
imagewin: everything about drawing on the screen with imagewin.cc the main protagonist
ios: needed only for the iOS port
m4: m4 stuff
macosx: what we need for macOS
mapedit: Exult Studio magic
msvcstuff
objs: how to handle objects in the game, generally and psecifc stuff like eggs, jawbone, spellbook, virtue stones...
pathfinder
server: the server/client stuff that Exult uses with Exult Studio
shapes: the graphics of the game
tools, various tools we have
usecode: important to handle the U7 scripting language usecode
win32
and then of course everything in the root. But it's mostly self explaining from the names.
Our explanation is that Exult evolved from a map viewer to a game engine to a fully editable game engine...