You can Google it, but AUM at scale means .03% is a significant amount of money. There's also uninvested cash that the broker can invest in t-bills and take the spread.
I bet the uninvested cash product drives some weird incentives - kpis around increasing ratio of sells to buys and increasing pain around removing cash.
Quite a lot of customers either have cash sitting in the account which they make interest on, or have margin debt which they charge for.