Also, read the prospectus of any ETF you already own, and you’ll get the gist of APs, enough to understand why your fees are so low.

There's also securities lending (to short sellers), many ETF sponsors retain all proceeds from securities lending for themselves. This is similar to how many brokerages earn most of their revenues by taking a spread on cash balances, so they can charge very little on transaction costs (i.e., they pay you less for unused cash than money markets pay them). After all borrowing money is the same as shorting cash, so there's nice symmetry here :)