But part of the point of D&D settings, right from the earliest days, is that the setting
doesn't support its own technological level. It once did - but from the earliest days if you wanted magic spells or magic items the best place to find them wasn't merchants or universities but in dungeons.
There are, in my experience, two basic Points of Light approaches. In 4e (which I believe was the trope namer) the Nerathi Empire fell only a couple of hundred years ago. The Keep on the Borderlands is a different approach to a Point of Light - and the Keep can't support itself. Few keeps can; the keep's job is to keep the interior safe so the farmland is protected and it gets outside support from the places it's protecting.
But we aren't talking pure medieval farming methods in a D&D setting. First there's magic;
plant growth literally doubles yields, all for eight hours of a spellcaster's time per year - and if most of the food would normally be eaten by the farmer and family that's a massive improvement in productivity. Second there's left over infrastructure from before the fall; just because no one's making aqueducts doesn't mean they no longer exist. Elves probably practice
wild farming. And there's evidence that Native American farming techniques were significantly more effective than European ones. For that matter one of the causes of Japanese internment in WWII
was apparently a land grab because Japanese immigrants were massively out-farming Europeans. I wouldn't consider it surprising in a fantasy setting if humans produced massively high yields by real world European standards - and were considered poor farmers.
Here I suspect that there's less of a problem in a D&D world than you might think; there are lots of fairly fast breeding sources of leather that you don't need to feed because they come to you. Such as worgs.
And here again we're looking at Points of Light. The best weapons and armour in an orthodox D&D Points of Light setting comes from dungeons. Good stuff can come from battlefields or armouries. The making of the equipment for legions was done centuries ago - and at least some of it is still fine. Also do you really need anything like so much wood when you have magic to heat your forges?
Again not so much an issue. If you're running a post-fall setting you have plenty of iron from the old empire. If it's a keep on the borderlands that happens back home.
No you don't need a quarry for a stone wall; you just need rocky outcroppings. A lot of dry stone walling for stone walls between fields were constructed out of "fieldstone" - or taking the stones out of the field (making it a better field) and using them to create the walls. As for paved streets, the fallen empire did a lot of that and you don't need so much to maintain the paving.
Or the Wall of Stone spell if we're using D&D tech
And part of the premise of the Points of Light setting is that the infrastructure used to be there - but this is one of the many threats to the local region.