“Frequently will see MM for million (thousand, thousand)”
Frequently? Roman numeral MM is a bit archaic. :-) Nowadays M is common practice for million with K commonly used for 1000.
Not on trading desks all over the USA. And it is used by the Wall Street Journal. I worked on the floor of the Pacific Stock Exchange, was head of the Equity Desk and traded at exchanges all over the world.
/shrug see it in business finance a lot still at least
The government has a handy table:
https://www.nist.gov/pml/weights-and-measures/prefixes
However, the real estate industry is a bit behind the times. They will use M for a thousand feet (meaning square feet — another sloppiness) and MM for a million. Supposedly, it's influenced by Roman numerals, where M means thousand. But MM in Roman numerals means two thousand, not a thousand thousand.