TD $0 commission on US online stock and ETFs seems too good to be true. Is there any hidden fees? How do they earn with this business model?

No, I am an TD Ameritrade U.S. customer, no important hidden fees, it works, they made their money (mostly) not by your former trading fees but by 'order routing'. for the U.S. markets at least a lot of other Singapore brokers should follow soon. there are with TDA some, mostly irrelevant, other than trading fees.

https://www.tdameritrade.com.sg/why-td-ameritra...

TDA and many other fee-free brokerages make money off $0 commission trades through order flow execution, where they receive a small payment for routing your order through a wholesaler. You can find this information in each broker's SEC Rule 606 disclosure report.

This may be disadvantageous to you, especially if you are executing large orders. In this case, you may want to consider other brokers such as IBKR Pro that do not receive payment for order flow execution and pride themselves on having significantly better price improvement than the industry average.