Don't people usually end making changes to the Unreal Engine executable and tie their game to it? As far as I know, it's not possible for me to take the Unreal

Don't people usually end making changes to the Unreal Engine executable and tie their game to it? As far as I know, it's not possible for me to take the Unreal Engine source code, compile it possibly with modifications, and play arbitrary Unreal Engine games with it (like say Gears of War 4).