> Where does the money the landlord pays in property tax come from?

From the landlord's bank account, hopefully.

It could have gotten there in any number of ways. Rent is the most obvious, but there's also the landlord's own paycheck, their spouse's paycheck, other investment income, social security payments, alimony and child support payments, gambling winnings, inheritances, loans.

> If that tax doubled next year, what would happen?

If it's a tight property market, rents go up. If housing is plentiful, or the local job market is in decline, the landlord eats the loss.

> Where does the money the landlord pays in property tax come from?

It comes from me.

> If that tax doubled next year, what would happen?

It would cut into his maintenance budget, utilities would break down, and I'd start paying rent into escrow until he ponied up.

Or he'd raise the rent by no more than 10%