The futures price is the spot price plus related expenses. Such related expenses include storage fees, management fees, interest on capital occupation, and so on. The capital interest, storage fee and management fee here are all your holding costs. If you buy a spot, you must store it before you sell it, which involves warehousing and management. If you want to sell it in a few months, the price must be higher than your purchase price plus these two expenses, otherwise you will lose money. Besides, if you put your money in the bank, there will definitely be interest. So if you buy a futures contract, at least the seller must be higher than the price at the relevant time plus interest to make a profit. This is the truth of futures. The futures contract price after three months is the spot price plus three months' storage management fee, plus three months' interest fee for the same funds deposited in the bank.
The purchase price of a commodity is fixed, and there are related expenses during the holding period after you buy it and before you sell it, which is your holding cost. The spot price is only calculated once.