I have a question about both of these codes: Where should I start to look to resolve each code and could one code possibly be causing the other?

F21 is a drain error,  F25 is tachometer error which is built into the motor and relays the spin speed back to the control.. Now if it's not draining properly and the washer is not spinning properly because it has water in then yes that error could be caused by the drain error. Clean the out the drain trap filter on the pump and if that does not help the drain issue replace the drain pump. Once the drain issue is resovled, if you still have the F25 issue replace the drive motor also. Both parts you need for that model are linked below.

Drain pump - https://prismparts.com/products/for-whirlpool-washer-washing-machine-water-drain-pump-pp0463693?_pos=10&_sid=29012fe33&_ss=r

Motor - https://prismparts.com/products/for-whirlpool-duet-washer-drive-motor-pp4481106?_pos=9&_sid=29012fe33&_ss=r