Depends on terminal. Look for signs for 'Arrival Pickup', but info counter should tell you.

At T3 is it Basement 1 (arrivals are at B2), at T2 - the same floor as a arrivals. T1 - have not used yet.

Uber app is smart enough to identify at which terminal you are and tell you where to go.

Also - at arrival pickup floor doors are numbered, so when you send message to driver, tell also next to which door you will be waiting.