All the major US brokers started doing free trades for stocks and etfs. For Vanguard, most of the index expense ratios are really low, like %.05 percent, but that’s not a trading fee.
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.
Even for paid transactions that typically give better pricing (IBKR Pro), the prices are extremely cheap.