Uncovering binary black hole formation mechanisms with gravitational wave detections

Gravitational-wave observations by LIGO and Virgo are revealing a population of merging stellar-mass binary black holes. Thus far, the broad connection between the inferred population and the physics that underlies black hole formation remains elusive. The O4a run of the LVK has more than doubled the number of binary black hole detections, and this new data is revealing new features in the population and elucidating old ones. In this talk, I survey several interesting and sometimes perplexing features and subpopulations in the population distribution. I show how they let us glean some information about the underlying formation channels and connect with transient and compact object astrophysics. As the number of detections balloons in the future, these subpopulations will be footholds to a more global understanding of binary black hole formation and the rich physics behind it.