When the world resets, the demons must conserve their strength to survive the long soulless winter. A demon lord from the last world may have consumed a million mortal souls, but then must wait eons before a new world is ready to feast on again. Like a bear in hibernation, his "fat" is depleted, and when humans return to the world, the demons are starving and weak.
Many demons will be too hungry to be interested in a long-game; so, they run madly into human camps looking for "scraps". These starved demons will be too weak to muster any magical abilities at all and the human settlements too small to create a meaningful supply of souls; so, until the human populations begin to grow, the demons will live like wolves just barely surviving off of field mice. Without the excess power to muster magic, the fighting will be spear vs claw.
At this point, the demons don't need technology, they need more people; so, those with more self-control will re-teach the humans about farming and animal husbandry to move things along. These human flocks will grow as their food sources become more stable. Now these more patient demons have hundreds or even thousands of humans in their flock. They can now muster minor magical abilities, but as the humans learn to make a plow, they also learn to make weapons. Now when fighting happens, it is between increasingly large armies of human with iron weapons and armor vs fewer but increasingly powerful demons armed with flaming claws and lighting bolts.
Soon, urbanization and industrialization will set in. Humans will form cities in the hundreds of thousands or even millions. The few demons that are left over from reset will now be becoming very powerful, their cults are massive, but they are outnumbered by a LOT! These demons begin to muster more magic than they know what to do with, and their herds become to vast to oversee alone; so, they use their excess of power to begin reproducing. The originals become demon lords, and their children serve them. Humans begin to develop muskets and cannons making a battlefield too dangerous for the demon lords; so, they just spawn hoards of minor demons to go to war for them. These demons may have minor magical powers of their own to contend with the improving human tech, but are never given enough power to challenge the lord who made them.
As the info age sets in, human technology explodes. Suddenly, they can make huge technological strides in a single generation, but all that ebb and flow of knowledge also gives the demons many new outlets into the human psyche. No longer needing to stand before a crowd in glory, a demon need only become an internet meme to go from a lowly peon, to a powerful lord. Whereas before, a it took centuries for a demon to become known enough to wield great power, now it can happen in weeks. At this point humans are wielding all manor of modern weapons, but greater demons with enough power to face a fighter jet or tank now number in the 1000s, and they add to their numbers every day has humans become more and more preoccupied with thoughts of the demons.
Then, all at once, Dea reaps his harvest and the humans are gone. Millions of demons with horrifying powers are left over without a single soul to consume. Many will flee to the deep places of the world to hibernate, others will begin to cannibalize each other trying feutilly to retain some shadow of their former glory. In the end, only a small handful will endure until the humans return to start the cycle all over again.
While the demons learn to use human technology to spread their presence, they have magic; so, they never bother themselves with learning how to build a railgun or a guided missile because they can just point their fingers and do just as much damage; so, when the world resets, there will be no "technology" to bring back other than the basics of what they plan to teach humans to help them become numerous again.
