is it free to use personal hotspot

The hotspot feature on the phone is provided by your cellular carrier. The hotspot feature might be included in your basic plan with the carrier, or it might not. In some cases, you will encounter extra data charges using the hotspot.

If in doubt, contact your carrier and ask them what exactly is included in your current plan....and what is not.

That would depend on your cellular provider. Apple charges nothing for the service, but your cellular provider may.

Check the data plan with your provider.

Many times, personal hotspot is included.