Impressive score!! The all-time high I've ever seen is 106000 (my wife's 4 hour playing frenzy). I think that completing the 4th floor would have required genius levels of skill, and/or miracle levels of luck. The crane doesn't scale well for higher floors currently because it moves too fast for higher floors. There were supposed to be optional clumsy elevator platforms and optional floor boards that you'd be able to build during later levels, but I didn't get time to do that during the jam. Also there'd be trucks and construction equipment that drive around, which you could use as something large and magnetic to step on. Oh well, maybe later.
The attraction system (ab)uses Unity's Particle System's collisions. Every second, hundreds of invisible little particles shoot out of the poles of the magnets and collide into magnetic objects only. Each particle collision generates a little pull between the object (at the point of the collision) and the magnet's pole, depending on how far away it is. See this tweet for a gif showing a debug view of the particle collisions:
It's not true to real physics, but geared more for clumsy fun physics. The real physics prototype (the thing I was salvaging in SalvageJam) was way too wonky for playability. There's actually a little magnetic pole in the butt of the magnet too that weakly repels very nearby metallic objects so that the magnet will be a little more likely to stick to things with it's head. And instead of varying attraction with an inverse square of the distance, it's using an inverse cube of the distance for a snappier effect.
The movable objects (I-beams, barrels, etc) actually have a speed limit on how fast they can move so that they don't get launched too crazily into the air because of the magnetic "slingshot effect" that may happen if you move too quickly. When a moveable object makes contact with the magnet itself, it just uses an invisible breakable fixed joint to simplify the simulation, and making it invisible to the particle collisions. This joint is broken when the force on it is too much, or the player presses jump, and the object then becomes visible to the particle collisions again.