We are not talking about giant steam facilities like those used by major industries we are talking about a small steam piston engine that could be used to run a generator or mill. That really isn't that complicated to make if you have the whole world to scavenge from. If you sent me back in time to the iron age it might be difficult to build a steam engine but with already the parts already laying around waiting to be assembled I could not it out in a couple of weeks. Even back in time if could get some crafts men and resources I think I could make a go at it.

OF course I wouldn't really bother with steam except maybe for maybe heating. If I wanted electricity I would find a warehouse with a propane powered forklift, very common, then I would gather some of the millions of propane tanks laying around in back yards and use that until the propane ran which could be decades. Find a natural gas power plant and they would have enough gas to last a small community for a long time.

The is some much stuff you wouldn't need to worry about making new stuff for a long time. You don't need the idiot savant to make bullets. We used to make them in my friend's garage when I was teen ager and there are still plenty of that kind of gear laying around in garages. If you get of the city the ability to fix and build things really isn't that rare.