Last night we arrived at the Cancun airport for our flight home. We had our negative COVID test results on our phones and printed out. We had already signed the Attestation form: https://www.cdc.gov/quarantine/pdf/Fillable-Attestation-English-p.pdf
We didn't actually need to use the form I linked above because Delta made filling it out part of our check-in process for our flight and then stored it to our confirmation code. They never asked anything else about the Attestation.
We did not have any bags to check but we still needed to go to the check-in counter, which another traveler at our hotel had mentioned to me or I would not have known. We approached Check-In and a uniformed Delta employee stopped us and asked for a paper copy of COVID test, along with our passport. He put a yellow sticker on our passports and then let us approach the Check-In agent.
The Check-In agent asked for our COVID test and passports again and looked everything over. We were never asked for the test results after that second check.
At security, we were required to fill out a Mexico-based form stating where we had been and affirming that we had no symptoms.
From there, the boarding process was just the normal deal and now we're back, awaiting our next trip:)