Just read it
I'll do a dot point progression.
- Stars formed earlier in the universes history has less metals (elements other than hydrogen and helium).
- Molecular Clouds with a lower metallicity will form statistically larger stars than the present day. They burn faster and hotter so a more "efficient" at making black holes.
- Globular Clusters are gravitationally bound so many of what originally formed is still there (except gas).
- Stars larger than maybe 15-18 solar masses formed black holes.
- Gravitational interactions in a closed system tend to force heavier objects towards their cores (black holes move towards the centre).
- Stellar mass black holes merge and become intermediate mass black holes.