I think this phenomenon has become a lot more important in the last few decades...I think it was fully possible for one person to understand how a car worked, how household appliances worked, etc, until quite recently

Probably depends on how you slice the question / define "fully understand." "How it works" in terms of being able to tell yourself a story that makes sense? Sure. Car burns gas, gas makes engine go, engine turns wheels. But what about the details you'd need to actually make a car (which include but are not limited to thermodynamics and metallurgy)? Probably not.

One difference is that there are parts of the system that, most likely, not a single person understands nor, even with weeks of effort, could understand. At least with cars, you can be reasonably sure somebody understands or can reverse engineer any specific component. With certain software that is a lot less possible.

It would be like if our society couldn’t fully function without Roman concrete, but no one knew how to make it anymore. Which is sort of what happened during medieval times: people continued to use Roman roads and Roman bridges. But if anything degraded, there was no hope of repairing it.