- They are making money on untouched deposits in two ways: through interest on deposits and making loans.
var idleDepositCash: Double
earnInterestOnIdleCash()
lendIdleCash()
sendOrdersToDarkPools() 
profit()
This idle cash can add up. Schwab clients’ accounts total about $3.7 trillion, according to the latest numbers as of the end of August, with an average of about $265 billion of it earning interest for the company. The money Schwab makes from loaning out its customers’ cash surged to 57% of its $10.1 billion net revenue last year. For TD Ameritrade Holding Corp., the figure last year was 23% of its total $5.4 billion in net revenue. At ETrade Financial Corp. it was 64% of $2.8 billion in net revenue.
- And order flow—sending your orders via third-party “flash traders”.
Isn’t RH doing this too?
And while these companies are no longer charging commissions, that doesn’t mean they aren’t making money from clients’ stock trades. A chunk of their revenue is what’s known as “payment for order flow.” Basically, the brokerages get fees for sending their customers’ buy and sell orders to computerized trading firms such as Citadel Securities and Virtu Financial Corp. Those firms then match buyers with sellers themselves. (The trading firms can make money by picking up the tiny spreads between the prices offered by buyers and sellers, or by trading on any gap between the futures market and stock prices.) Brokers and the trading firms say this process results in a better deal for retail investors.
TD Ameritrade is probably affected the most by zero commisions:
Source - Brokers Profit From You Even If They Don’t Charge for Trading - Bloomberg