Or the hardest part are the 3D printable parts?
The hard part is not the ventilation mechanism itself, but the specifications. Those ventilators must be controlled precisely, they must take into account the characteristics of COVID-19 patients. There is some overlap with ventilation for ARDS. The more you read into this subject, the more you realize how absolutely scary it is to actually design such a project. Lots of parameters have to be taken into account.
Read on mechanical ventilation. https://en.wikipedia.org/wiki/Mechanical_ventilation
3D printed parts that where the air flow through must be air-tight.
Do you really want to give these people a new ventilator with which they are not familiar, which may lack features they need, and which might break down or malfunction in an unfortunate moment?
Maybe there is a place for such simple ventilators outside intensive care, e.g., as support for people who are able to breathe on their own and do not necessarily require mechanical ventilation, AND who are in a place without nasal O2 supply?
Every ventilator manufactured can provide blue prints of their old designs. There has to be old design without microcontrollers that could be made in most machine workshops around the world. I guess a modern design would be to complicated to outsource to random machine shops with all the special electronics.
I don't know if they make them in-house, through a supplier, or both, but they have the capacity to not only get the things manufactured but deliver them too.
Nor should there be. Centralization isn't beneficial for this sort of problem.
Edit: It might contain electronics, but there is pandemic, not global war, so printed circuit manufacturing capacity is still there. Some modules with microprocessor are widely available worldwide and the design can be constrained using such module and 2 layer carrier printed circuit board. Machines to build these are available at better maker spaces. Pick and place machines are also there. So basically my timeline would as follows: order parts on Digikey and mouser today, go to maker space, start engraving printed circuit boards and launch 3D printers. On Monday or Tuesday digikey ships the parts. Maybe it will arrive on Wednesday, shipping companies are busy right now. On Wednesday I would launch pick and place machine and produce first batch of boards and next day I would start assembling manually the devices. On next weekend I would have probably 10 units to start tests. That’s working alone. With other tech savvy individuals output would be orders of magnitude higher. And this is not theory, in my previous workplace team of 8 blue collar workers was shipping 2000 units of comparable machinery per year.