Ann: Fabrication Agreement with Earth Mountain, page-196

I just found this by holding mouse over Technical Team photos.

Mr Liu leads a team of 15 experts who work closely with Audio Pixel Ltd experts toward the implementation of Audio Pixel technology in earth Mountain Mems fabrication and packaging lines.

So it looks like they have been working on fabrication of the mems chip itself since 2018. I now have a vague memory of AP mentioning multiple mems vendors at some point.