that monster looks amazing. unreal lighting is very impressive too.
everything uses constraints, no line trace suspension or anything like that. i could make things float but i dont want to mess around with getting decent water interaction so i am avoiding water altogether.
you can connect any object to any other object with any type of constraint, hinge, wheel, spring etc.
objects with large surface areas, such as a door, have basic aerodynamics for use as wings. things like fans can be used as thrusters. engines can rotate wheels.
its possible to make pretty much anything including walkers.
the early prototype for the core mechanics in unreal engine was this [Android] Physics Construction Set
the game is also about exploring the environment to find the parts to achieve missions.