There is exactly one manufacturer of large EPDs, E Ink Corporation.

A few have tried to compete over the years, none have really managed to ramp up production and compete from my understanding. I believe there are some chinese firms making smaller panels now, mainly used in electronic shelf labels for stores.

From my understanding most of the patents for the foundational tech have expired at this point. I believe the panels are just generally very difficult to make, with difficulty ramping up further as panel size goes up, presumably the yield is fairly low.

That was part of the motivation here: If I can think it up, can AI actually build it, even when it's outside my day-to-day experience? (and can it do it without frying the hardware somehow)

The last time I checked, patents.

there's no secret, they're just expensive to manufacture