The “Dark Ages” is just a term that refers to an bit of time where we have few useful records, because those records were lost. Even Italia wasn’t all that “dark” for very long, and the rest of Europe mostly went about their business.
But as to the OP, the cantrip Mold Earth exists, as does Shape Water. Medieval European irrigation doesn’t exist, the irrigation is vastly better than that.
Control Flame helps with smelting and other heat-reliant tasks. D&D has common magic items, that can do cantrips 1/day or more, or mimic effects of cantrips.
And that’s just to start.
When I run a Points Of Light game, the points of light are wonders. They are the bastions of a lost world or the first great wonders of what will someday be the early history of a world, so things like hanging gardens growing much of the food, supplemented by semi-domesticated fowl, hunting parties bringing home the deconstructed carcasses of megafauna, fishing boats, and trade with the small farming villages close enough to be protected by regular patrols, where every able bodied person is a trained combatant.