Profit on Residual Cash

I hear a lot of people talking about how Robinhood and Venmo make money on the cash that's sitting around in their accounts. My research hasn't shown that this isn't what either company is emphasizing. I'm curious if anyone is aware of numbers of how much it's worth to a platform to have this type of cash sitting around. For example, how much money could a platform make on $1,000 in a month (assuming there were user friendly agreements and the cash had to be liquid like it need be for these companies).