How does Apple Offer 4.15% APY?

It's actually a Goldman-Sachs account in Apple packaging. The federal funds rate is in the 4.75-5% range, which means Goldman, in theory, can take the money you deposit and re-lend it at 4.8% or so, pay you your interest, and still make a profit.

As others are saying, the high federal funds rate makes this quite easy. Vanguard offers a Federal Money Market that can be used as a cash-sweep in its brokerage accounts that's currently offering 4.88% compounded yield (VMFXX). I'd rather use that myself than Apple's lower 4.15%.

Also, probably somewhat of a loss leader, getting folks to move their accounts.

3-month treasuries pay 5%.

they don't, it's a branded account. wealthfront's cash management account currently pays 4.3% up to 3m.