fallout-4
One of the strangest traps in Fallout 4 is the motion-sensitive cymbal monkey trap: a toy monkey that starts playing the cymbals when you get near it, with a trigger attached to it.
You can't pick these up, or disarm them for parts, or anything. But is there a way (possibly a mod) to make your own and set them up in your settlements?
Traps work exactly as you'd expect - basically in an identical manner to lights and other electrical devices. If you've had a pressure plate fall through the floor I'd call this a bug. It's likely it didn't agree with you placing it exactly on the floor and some clipping issue occurred when you stood on it.
The other thing I came across is that you can effortlessly break these traps (especially the tesla trap) just by having it on. To repair a broken trap in Workshop mode, press E with the item selected.
The things to notice are only the "metal" section of the pressure plate depresses, when it does the red light goes green. When there is no pressure the green light will go red. While the light is green, power will pass to connected devices. While the light is red, it won't.
You can craft ammo with the Contraptions Workshop DLC using the Ammunition Plant.
I have yet to try this in-game myself, so this answer isn't very detailed, but I'm surprised no one has written a nice answer about it yet, so let me at least note it.
Best Answer
As far as I know, I've never found a way to build these in the vanilla game. They are used for traps as you said or alerting nearby enemies when you come across them in the Commonwealth. The Wiki doesn't say you can build these either.
I don't know if any mods exist for being able to make them, a quick Google search doesn't seem to yield any promising results.