Imagine we nuke the world right now ... How long until we can start creating a modern iPhone again. Right now only 2-3 countries are able at all to produce these chips. And all rely on very sensitive technology.

Knowledge might not be lost but rebuilding tech might take centuries depending on how worse it is. And that is probably very similar to the past. Middle age people probably understood how Romans build their water supply but where unable to reproduce it because how the bad the time was.