They could get a percentage of the gate receipts as a payment for using their facilities.
They could also get a flat fee from the FA and not get a penny from the gate receipts.
Given that the charities committee deemed that the shield game could not be classed as a charitable event as not enough of the gate receipts went to charity, we can deduce that 100% of the gate receipts do not go to charity.
Therefore, it is reasonable to assume that stadium rent is taken from those gate receipts.
I don’t really understand why you can’t understand this.