Federal law requires that you report money of over $10,000US when entering the USA, by land, marine, or air. It is a reporting requirement. Failure to do so can cause forfeiture of the money if you are caught.
"Money" means monetary instruments and includes U.S. or foreign coins currently in circulation, currency, travelers' checks in any form, money orders, and negotiable instruments or investment securities in bearer form.
The same $10,000US applies when entering Mexico.
The report does not trigger any tax.
https://www.cbp.gov/travel/international-visitors/kbyg/money