Found in 1 comment on Hacker News
A whole lot goes on behind the scenes with securities trading, your interface to it as a retail trader buying and selling is only the tip of the iceberg.

Most of the people talking about various conspiracy theories about brokerages have clearly very little idea of how brokerages work on the back end (and not very many people do know this) or what regulatory duties brokerages have and hwo that might inform decisions they make.

If you are a bit of a masochist, you can read all about it in After the Trade is Made

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