They partner with a company that has an existing device. They secure additional suppliers for each part of the device, to scale up. They pick an ideal factory to use, one that has existing clean room capabilities.
It will take months to fully ramp up, but not to churn out the first few thousand units. Ford + GE Healthcare think they can hit 50,000 units over 100 days. If Ford were working alone, it would take a lot longer, they would have a lot of wheels to reinvent.
The US, Europe and China may need to flood the world with ventilators over the coming year, depending on how things go. Hopefully the US ramps to a level of considerable surplus production, rather than letting up.
https://www.reuters.com/article/us-health-coronavirus-usa-fo...
"Ford Motor Co said on Monday it will produce 50,000 ventilators over the next 100 days at a plant in Michigan in cooperation with General Electric’s healthcare unit, and can then build 30,000 per month as needed to treat patients afflicted with the coronavirus. "