At this point we all have a copy of that, I would think.

right at warthunder.com

Source? I assume everyone has the unclass info but I can't find anything about China getting the classified deets.

Because no one on either side would want to admit it?

For example, this story was in the news back the day for a while.

https://www.securitynewspaper.com/2021/08/23/how-chinese-apt...

I see no mention of classification levels so I will assume unclass.

Why aren't these things developed completely air gapped?

Because you cannot coordinate activities across multiple sites and suppliers if they were.

How did we do that before the internet?

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.

Because Microsoft Windows cannot be airgapped.