Generally cash real estate transactions aren't subject to most KYC / AML rules. But the US federal government recognizes that as a problem and is working on tightening enforcement.
https://legal.thomsonreuters.com/en/insights/articles/u-s-re...
https://legal.thomsonreuters.com/en/insights/articles/u-s-re...
That's not entirely true. The bank wants to know enough information to make sure that the cash came from the buyer, and not from someone else other than the buyer, for AML reasons as well as creditworthiness. So they do seek proof of the provenance of the cash sufficient to demonstrate that it actually came from the buyer.
(Source: detailed conversations with bankers when buying a home.)
https://www.washingtonpost.com/archive/realestate/1989/05/06...
(family member is a mortgage underwriter)