When using facetime on an Apple iPad, does it use wifi or your cellular data?

That depends.

If you iPad is connected to Wi-Fi it will use Wi-Fi.

If you are not connected to Wi-Fi but have cellular service on the iPad it will use cellular data.

If your Wi-fi is weak, and you have Wi-Fi assist turned on, it may compensate with cellular data.

link-> About Wi-Fi Assist - Apple Support

It would use WiFi as a first choice, or cellular data if WiFi is not available.