Private prisons in the US charge the government a fee per prisoner per day. The government pays it with tax money.

The US Constitution allows slavery as a punishment for crime. Private prisons can force prisoners to work in prison shops, or rent them out to businesses, and punish them if they refuse to work. The prisons pay the prisoners less than a dollar an hour and make enormous profits off their cheap labor.

Let me emphasize: this is slavery. This is actual literal slavery. The US government, and the private prisons it contracts with, collectively own almost two million imprisoned slaves, the majority of whom are Black. Slaves who refuse to work are punished with solitary confinement or loss of "privileges" like food, hygiene products, or communication with friends and family outside the prison. United States law and the United States Constitution allow it. And slavery is extraordinarily profitable for the slave owners.

Finally, private prisons are allowed to charge prisoners more or less whatever they want for things like phone calls and purchases from the prison commissaries. A prison can charge $5 for a 15 minute phone call or $10 for a bottle of shampoo. Since prisoners, again, earn less than $1 an hour, these ludicrous fees are generally extorted from the friends and family of prisoners, providing another revenue stream for the prison.

All this is so profitable that private prisons' contracts with the US have minimum occupancy guarantees. The US government guarantees it will supply a certain number of prisoners to each private prison to guarantee its income. Somehow, despite crime rates falling consistently for decades, the US government never fails to make quota.

FYI, the private prison industry was one of Trump's biggest donors, and their investment paid off massively - the industry is making billions from imprisoning and enslaving immigrants thanks to Trump's immigration crackdown.