1) If you get a new device and use your old device's recovery phrase on the new device, what do you do with your new device's recovery phrase? Do you keep both recovery phrases? Or which one to keep?
In the beginning, you are asked if you want to create a new wallet or restore an existing one. You won't get a new recovery phrase if you choose the latter option.
2) Can't you just send the crypto to the new wallet instead?
Of course, you can. However, that could involve a couple of transactions if someone used multiple accounts or coins.
3) Someone mentioned resetting your old device back to factory defaults and selling it. Does the resetted device generate an entirely new recovery phrase, or the same one? I personally wouldnt buy someone's old device.
Yes, it does. Otherwise, you would have to buy a new device if you somehow exposed your seed.