C:\Program Files (x86)\Steam\steamapps\common\<GameName>\installscript.vdf Sometimes it resides in a subdirectory like _CommonRedist or directly in the game root. It is present for all games—only those requiring custom install logic.
"HKEY_CURRENT_USER\\Software\\MyStudio\\MyGame" "SaveDir" "#InstallPath\\Saves" "ConfigVersion" "2" installscript.vdf
"Run Process" "DXSetup" "command" "#SteamPath\\steamapps\\common\\MyGame\\_CommonRedist\\DirectX\\Jun2010\\DXSETUP.exe" "parameters" "/silent" "WaitForProcess" "1" "VC2015Setup" "command" "#SteamPath\\steamapps\\common\\MyGame\\_CommonRedist\\vcredist\\2015\\vc_redist.x64.exe" "parameters" "/quiet /norestart" "WaitForProcess" "1" C:\Program Files (x86)\Steam\steamapps\common\<
"source" "#InstallPath\\data\\settings.default.ini" "dest" "#InstallPath\\settings.ini" installscript.vdf