In order to scan/fax from IPhone 12 Pro Max, what app is needed or suggested?

This is the app you are looking for:

https://apps.apple.com/us/app/hp-smart/id469284907

I would recommend that you use the app that fits your scanner. You do not tell which brand it is, but if it is a Canon, for example, search for Canon Print on the App Store. You can possibly look up the printer model on the manufacturer's website and see if they have made an app for the specific model.

If you want to scan a document directly from your iPhone, then you can use the Files app.

Open the Files app. On the Browse screen, tap the circle containing three dots button at the top-right corner of the screen. Select Scan Documents and use your iPhones camera.