If I use my US iPhone wifi in France to use my iPad is it free to use

Are you asking about using the Hotspot feature on your phone to provide a Wi-Fi signal for the iPad?

If yes, your phone will have to be able to connect to a cellular network in France....and....the cellular carrier must provide a Hotspot feature with their service.

You should expect to pay for data use on the phone, so things are not going to be "free".

Check with your current carrier to see if they offer an International Plan so you don't have to try to set up service with a carrier when you get to France.

If you are asking if your iPhone and iPad can both connect to "free" Wi-Fi in France, the answer would be yes.

If you are going to connect to a Public WiFi then it may be free.

If you are referring to WiFi Calls, get in touch with your carrier that provides WiFi calls.

WiFi Call is a Carrier Specific feature. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you. --> Make a call with Wi-Fi Calling - Apple Support (IN)