When my iphone is connected to wifi will personal hotspot use wifi instead of cellular data?

Hello, My laptop recieves a very weak connection on my home wifi network however when I use my iPhone as a personal hotspot in my house it has a strong connection and works very well. My iPhone is also connected to my home wifi network but picks up a much stronger signal.

I would like to know if the hotspot connection (when used at home) uses cellular data, or because the iPhone is connected to wifi does it use this instead. I am asking as I do not want to run up a huge cellular data bill with my network provider.

Many Thanks

Alex