My samsung galaxy s21 ultra had a nice touch with always on display clock with gift images but since a recent update you can't change the clock on the always on display and I know you can change the clock in home screen and lock screen it's the Always on display they messed with I would like that back the next time they decide to update
Download Good Lock app from Galaxy store and install the Lock Star module. It will allow you to customise both the AOD and lock screen. Also check out the ClockFace module.