During WW2 in the United States, you had all sorts of consumer goods companies reorganized to output a prodigious amount of military supplies. There were multiple companies making the same model of things, with fairly rigorous QA to ensure quality and uniformity.

For example, the BC-348 receiver, widely used in aircraft, was produced initially by RCA, and eventually "farmed out" to 3 other manufacturers.

More than 4 million M1903 Springfield Rifle were produced by the Smith-Corona typewriter company.

Here's a really good example, look at how the production of proximity fuzes, was distributed.[1]

The key thing is to have second sources for everything. Something the US military seems to have forgotten, or decided to ignore in their pursuit of gold-plated weapons systems that give the most kick-backs.

[1] https://usautoindustryworldwartwo.com/vtproximityfuze.htm

It's not a great comparison because Germany could not hit the US mainland. Even if there had been a single giant everything factory it wouldn't have mattered.