The USP of this deal is that it's the PRO-MAX version. If you are good with any Iphone, you are better off getting the Iphone-12 through Visible(for $400) or the even 13-Mini through other carriers for a lower price.
But if you specifically want the MAX version, such deals are hard to come by. As of today, Used/Refurbished versions of 11-Pro-max sell for upwards of $620 on Swappa and Ebay.
To purchase the device, you need to either create a new ATT/Verizon account or use your existing Account.
After activation, you can pay-off the remaining installments and move to a different carrier. However, if you need the device to be unlocked, you need to stay for 60 days.