The theory is that during the immediate aftermath of the Big Bang, the distribution of mass-energy became chaotic, with local hotspots where mass-energy was more dense, and other spots where it was less dense.

GR says that if ANY amount of mass-energy comes to occupy a small enough area, then that is a black hole. Star collapse BHs have a lower mass limit because their own gravity is what forces their mass into a region small enough to become a BH.

So primordial BHs could have formed with arbitrarily small mass, because they didn't have to rely on their own gravity to attain BH density. The Big Bang did it, for them.

https://en.m.wikipedia.org/wiki/Primordial_black_hole#Theore...