Schwab and Ameritrade Cut Fees to $0.

There's no such thing as a free lunch. Brokerages make their money from net interest margin, just like banks do. 

"...Schwab pays accountholders a little bit of interest on their cash balances (0.27 percent, on average, in 2018) and earns a lot more interest by lending those balances out (2.57 percent, again in 2018)."

"...They are able to offer "free" robo-advising because their robot will put a substantial fraction of your assets in lucrative-for-Schwab cash deposits, typically 6 to 10 percent but sometimes as much as 30 percent. If using this "free" robo-adviser will lead to you holding more cash (and giving up more yield) than you intended."

http://nymag.com/intelligencer/2019/10/schwab-and-ameritrade-cut-fees-to-usd0-whats-the-catch.html