We didn’t need an internet air gap before the internet.

How did we coordinate activities across multiple sites and suppliers before the internet?

Basically, I don't think enough people are seriously questioning the costs vs benefits of using the internet while dealing with sensitive data. You should assume that basically anything which touches an internet-connected device will eventually be stolen, and plan accordingly.

Before the Internet, aircraft designs were far simpler and thus it was feasible to do the work at only a few sites. The JSF / F-35 program then went even further by intentionally spreading work across many US Congressional districts plus several allied countries in order to make it politically impossible to cancel.

On paper and spies were just as effective back then.

Slowly and inefficiently. Projects like the F-35 are insanely complex and could not have been done on pen and paper.