Can I use iPhone wireless wifi to upgrade my software?

I don't know. You could try but possibly your device might know you were using a hotspot. Even if it lets you, you might end up with a corrupt file and have to do it all again. There's a reason why they require a more stable connection.

Click on a link below for full instructions about how to update with wi-fi or 5G, or using a computer if you have storage or connectivity issues.

"Update your iPhone, iPad, or iPod touch" click here --> https://support.apple.com/HT204204

Or this video:

"How to update your iPhone or iPad" click here --> https://youtu.be/csJAEyvvruY

I should say the use of the Mac as a go-between is the key for 4G network. I have used the hotspot of the actual phone being updated connected by USB The update file downloads then during update the internet is lost but update continues.

HOWEVER as Limnos pointed out if the update fails you may be completely blocked out from the second attempt. It is for the experimenter, not for everyday use.

I have tried using iPhone hotspot to update other idevices directly but that IS blocked by Apple.

only if you connect a computer to the other phone by personal hotspot using wifi, and then use the computer to update your phone. Some network providers will block it particularly American ones.

Network providers are not the ones that block upgrading via cell data, it is Apple.

FWIW I have done it many times in UK using my Apple Mac and my idevices using hotspots on iPhone7 and iPhone 12.