This is the main directory.
In 3DGE binaries, you will find multiple folders, with the newest copies near the top of the tree.
The latest release is 1.36.3, with the updates to the engine marked by letter (A, B, etc). You need a valid DOOM IWAD to play the game, or go grab a free, legit copy online (by the name 'FreeDOOM').
The /SOURCE folder is for the source code packages, which are compressed in 7z format.