I've been following the developments of these ventilators pretty closely, particularly of these using a ventilator for multiple patients.

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.