How brokers make a profit?

Is it when I lose money? or do they only making a gain on the spread?

Because I see that most brokers don't take commission.

Also if they only gain on the spread, how can I know how much my broker make on a single trade?



I want to know that my broker don't want me to lose my money, If they gain money on every trade that I make I will definitely know that they want me to make money so I will continue trading.