One thing I'll mention regarding BlackRock, StateStreet, and Vanguard is that they all own each other too. They're really not separate entities at all.
Megacorp owns everything. Slyver12 has put together a pretty good discussion of how that works, including an iterative treemap of financial ownership of Megacorp which you can find here. I think you'll find it instructive. I did.
Thanks!
Thank you for the extra details, much appreciated!