To accommodate limited hard disk space of the era, Lego Island by default installs only a few files locally and streams the rest off the CD. The locally installed files are as follows:
The main executable. Creates the window handle, initializes most of the DirectX libraries, and also runs the main game loop. A fairly simple application, it largely passes off to the functions in LEGO1.DLL for the bulk of the game code.
The bulk of the game code and vast majority of the functionality. ISLE.EXE can be largely considered an entry point into this library. Due to its importance, much of the modding and research work is centralized on this library.
CONFIG.EXE and LEGOISLE.EXE
Bizarrely, these two executables are completely identical despite being used for different purposes. On startup, both of them will check their filename. If it's "CONFIG", the Lego Island Configuration dialog is shown. Otherwise, it launches ISLE.EXE and exits. Their functions can be changed simply by renaming them.
The exact purpose of using LEGOIsle.exe instead of linking directly to ISLE.EXE is unknown, as is purpose of having the two be the same executable. All desktop and start menu shortcuts to LEGO Island actually launch LEGOIsle.exe, despite the fact that its only action is launching ISLE.EXE.