During the first second of the universe, space was a dense, hot "soup" of particles. The theory of PBH formation relies on Cosmic Inflation.
Quantum fluctuations during inflation caused some areas of space to be slightly denser than others. These are in fact massive spikes in energy.
If a region was dense enough (crossing a critical threshold), the internal pressure couldn't stop the force of gravity. It collapsed instantly into a black hole.
PBHs are unique because their mass is determined by the "Horizon Mass" at the time they formed. The earlier they formed, the smaller they are.
c equals the speed of light, t equals the time it had formed, and G equals the gravitational constant of the universe.
For example, If a PBH had lived for 1.4×10-25 seconds. Then it's radius would be 50% of that of a Proton but 560 billion kilograms. To put it's mass to a more understandable scale, if you were to combine all of humanity into 1 mass. It would be roughly equal to a primordial black hole 50% the width of a proton.
New data from the James Webb Space Telescope (JWST) has revealed massive galaxies existing far earlier than expected. Some theorists suggest PBHs acted as the "seeds" that allowed these galaxies to grow so fast.