can I buy an XsMax in the USA that is UNLOCKED?

Yes, but it's a bad idea to do so if you intend to use it in another country. You will not be able to get it repaired or replaced without physically bringing it back to the US.

Also, the ONLY source for unlocked phones in the US is directly from Apple. The phones sold by other retailers are not unlocked.

The 8+ you bought was used. You did not buy it from Apple and you did not buy it directly from Amazon. You bought it from a 3rd party. What you purchased through Amazon is not relevant to the question you asked.

In the US, the ONLY source for unlocked iPhones is directly from Apple.

The warranty is not and never has been international. It may not make sense to you, but there are financial legalities of transferring assets that can make cross border cell phone replacements problematic as well as differences in technical specs for phones sold in different regions.

You don't have to believe me if you don't want to, but not doing so isn't going to alter reality in any way.

Amazon is not an Apple Authorized reseller of iPhone’s, nor are any Amazon private sellers. And whomever on Amazon was selling them could have gotten it anywhere on the planet iPhones are sold, since it was a used device (even if sold on the original packaging, it had to have come from Apple or an Apple authorized seller somewhere originally).

iPhone warranties have always included the clause:

“IMPORTANT RESTRICTION FOR iPHONE, iPAD, APPLE TV and HOMEPOD SERVICE.

Apple may restrict warranty service for iPhone, iPad, Apple TV and HomePod to the country where Apple or its Authorized Distributors originally sold the device.”

https://www.apple.com/legal/warranty/

Since the only repairs are display and battery replacements, anything else will require a replacement, which will assuredly be cause for Apple or their Authorized service provider to refuse service. They won’t have the foreign device model in their service unit stock, and Apple’s arrangements with its manufacturers is that service replacements are always identical to the model and configuration that is turned in for exchange.