Well, the zener is the core component, but you need a driver and sampling circuit around it. Did you count the components or the pads? I count roundabout 20 components (~40 pads). The design by betrusted.io even manages to work with 14 components; look how tiny it is.
yeah that one is tiny but i'm sure if I tried to build something like that it's going to be way bigger. think breadboard size. because that's how you would have to get started is by breadboarding it up and seeing if what you constructed works and then once that passes muster, you can solder everything up. gonna be the size of 2.5 inch ssd most likely before you over and done with it. nothing wrong with that though i guess. bonus points if i dont have to actually understand how the thing works to build it...

That just confirms that this is a good circuit.

I would imagine all such usb devices use that technology in some way. They're definitely not sampling radioactive decay or something right?
I just wasn't aware that they're sold under the simple term 'TRNG', as I couldn't find anything when looking for 'avalanche noise circuit PCB'.
Because they want to be a bit cryptic about how exactly their device works. They don't want people to build one themself necessarily.

It would be good if these devices came with schematics and board files to verify the circuit more easily.
yeah there's no way that's happening with something like TrueRNG.
(1) Compare product to PCB files
(2) Compare PCB files to schematics
(3) Check schematics to understand what the circuit does and verify that it's what you want it to do
They don't want you doing that. No one want someone doing that to their product? The reason is simple. if you could do that, you could just build the thing yourself. and don't need to buy it from THEM.
Plus, presumably they've put in some R and D on the thing with some tweaks to make it better than the old off the shelf github circuit to give their product a competitive advantage in the marketplace.