A look at how Citadel grew from millions to tens of billions by acting as a middleman on stock trades — good for Citadel, less so for small investors on apps like Robinhood.

• Citadel makes money as a market maker: it buys and sells stocks and pockets the tiny gap between the buy price and the sell price.
• Because it sees all the order flow from apps like Robinhood, it knows what regular traders want before they get it.
• That direct access to the exchange lets it grab the spread on huge volumes of trades, which adds up to billions.
• Its business exploded during the COVID and Robinhood trading boom, when everyday people were buying and selling like never before.
• Anyone with enough money could do the same — being a market maker mostly comes down to having the capital and tight pricing.

Outlook: As long as small investors keep trading through free apps, market makers like Citadel will keep earning off the spread.