It doesn't start or end here, but
Unit Scripts contains some information that may be considered 'unit AI', most notably things like
Don't fire at radar dots and overkill prevention. That said, most of a unit script file should be considered the mechanics of how a unit works, like turret turn rate and death animations. As for unit AI, do you mean the unit AI toggle, or what units do on maneuver position and fightmove, or both?
unitscripts/unit defs is in Lua, noted by the filename ending in .lua
