In Samsung Galaxy S21, it vibrates when you put it in charging, this is because this setting is on by default.
If you want to turn it off then follow the below steps:
Step 1: Go to the Settings.
Step 2: Sound and Vibration.
Step 3: System sound vibration control.
Step 4: "Charging" Tap on it and off.
Note: Vibration setting is on by default, tap on it and turn it off. Jump down here Charging setting is on by default turn it off.