Earlier quoted context omitted.
The type of ventillators required by the most serious cases are much more complicated: as I understand it, it involves push a tube down to the lungs, and the machine breathing instead of the patient, i.e. it has to carefully monitor and regulate the pressure etc. Already the "push tube down to the lungs" part is quite a challenge: avoiding damage to the vocal cords, or to the lungs, etc.
Tubes down the lungs is definitely a bit tricky, not all patients will require that model though. As for careful monitoring, that is something the machine does on its own, as long as it has been configured correctly for the individual patient.
Some patients might need supplementary oxygen delivered through a canula, or through a mask, but that's nothing like the procedure used for a vent.