There are really a lot of those projects. I have been following this one:

https://www.oxygen.protofy.xyz/

It's open source and there are two versions: industrial production and 3d printing.

I also find the mechanical way it adapts to different patients interesting: the different breathing patterns are codified by different levers in the mechanism that push the AMBU. So, the programming is done changing a piece in the device.

There is this cool visualization too: https://protofyteam.github.io/oxygen-visualization/