Not sure about about cracked glass, but for devices in relatively good condition the Apple official site offers slightly better trade-in prices than Switch or Machines. Apple@TRX offers should match those, at the very least. You can try the trade in on the online store first to get a ballpark estimate. On the site store, buy a new device like the 16 Pro and then after selecting colour and storage you should see an option asking if you want to trade in. Enter your 15Pro’s serial no and then be as honest as you can with the questions asking about damage.