This will help you understand it better: https://forums.zwift.com/t/how-does-zwift-determine-my-speed
Also, what trainer, what trainer selected on Pairing Screen, sensors, tire size selected on Pairing Screen.
DO NOT MANUALLY CHANGE THE RESISTANCE ON YOUR TRAINER, KEEP IT AT THE SETTING WHICH IS DISPLAYED FOR THE TRAINER ON THE PAIRING SCREEN.
Tip, ignore the speed that the Fenix is displaying, it will hardly ever match Zwift.