How to get a QR code to activate my cellular data for an iPhone 14 Pro Max?

If you are referring the the setting up of an eSIM on your iPhone contact your carrier for a QR code, then follow Apple's instructions in Set up cellular service on iPhone

As stated in the article, a valid QR code has to come from the cellular carrier you will be working with. Unless the phone was purchased unlocked/carrier free, the carrier needs to be the one the phone is locked to.