hotspot data

If you wish to use your Hotspot, you are going to just need to turn it on. You will connect to it with the MB if you select the phone from the wi-fi list on the MB. If it will not connect, then Hotspot isn't working correctly. You should see the name of the iPhone appear in your wi-fi list on the MB. If they are logged into the same Apple ID, it should appear as part of the Instant Hotspot, and you can connect automatically. It will be using your cellular data, and may ask for the password if they are not logged into the same Apple ID. See this support document for using Personal Hotspot. How to set up a Personal Hotspot on your iPhone or iPad - Apple Support

Your iPhone will not send data over your phone's wifi connection to your home internet when a client is connected to the Personal Hotspot feature on the phone. In fact, if you temporarily turn off your Cellular Data connection, you cannot use the Personal Hotspot feature.

If it makes you more comfortable, you can turn temporarily disconnect your iPhone from your home wifi and then enable the Personal Hotspot for your other devices to connect to. This should reassure you that your devices are reaching the Internet via your phone's data plan and not your home wifi.

Hotspotdummy Said:

"hotspot data: [...]I am not sure therefore if I am using my Phone Data or the WiFi connection. will the iPhone always default to WiFi or is there a sequence involved when starting a hotspot so my iPhone ignores the WiFi and only uses mobile data.[...]"

Cellular Data Overrules WiFi:

Go into Settings and see if Cellular is enabled. Note that Cellular always overrules WiFi. So, as long as both Cellular and WiFi are both enabled, WiFi will not be used. So, enable WiFi and disable Cellular and all would be fine to run off of WiFi only.

Really, it seems to me that they have asked about connecting the MacBook Pro to their hotspot and not use their current wi-fi. That is what I just explained them to do. The Hotspot will not use their current wi-fi if they connect the MB to it, it will only use cellular data for Hotspot.

But instead they were asking whether their iPhone will be using their home wifi (which is slow) to connect to the Internet or if their iPhone will be using its cellular data connection for that purpose. The answer to that question is that the internet connectivity for devices connected to the personal hotspot will be device > to iPhone > to internet via cellular.

This wasn't what they were asking.

And I said the iPhone will be connecting to the internet using cellular. Then the MB connects to the phone's hotspot via wi-fi.