I have updated information from my call to AT&T Tech Support Android Division minutes ago.
Important Note:
I did not get this information from the "Call Center" but the Actual Android Tech Support Dept within AT&T Mobility Engineering Area.
This is the answer to how, when and who will push out Android OS Updates, bug patches, and increasingly critical "Google Play Services Udate" software modules to AT&T Nexus 6 models.
Bottom line: All updates will be routed from Google through AT&T for evaluation and approval, and then to the Customer.
Just like the past and present practice, there's neither a timeline, or assurance, as to what or when customers will receive any or all updates.
My opinion?
If your interested in the pure Google experience that occurs via having your Nexus device running the latest OS updates, bug fixes, et.al. from Google, buy your Nexus directly from Google via the Google Play Store.
Last year I bought my Nexus 5 and several more for family members directly from Google Play. A total of six in all... it's been the most fun, fast, trouble free group of phones we've enjoyed ever.
Good Luck with your decisions and future Androids