You can purchase a 3-day subscription to acdelco's "General Motors Service Repair Manual Information" and the spend a few hours with print-to-pdf.
www.acdelcotds.com
This is the official mechanism for GM Fleet, and it should contain the most up-to-date information.
Your owners manual also provides an official source for service manuals, specifically for those interested in doing DIY work. (Note that the helminc.com web server appears to be down for maintenance this weekend, but google cached the site as recently as Feb 12.)
NOTE: I haven't purchased either of these....yet.
For my 2015 Ford Escape, I own a Haynes manual as well as a custom ordered manual specific to my VIN. The Haynes manual covers a lot, and presents it in an easier format. However, the official custom-to-VIN manual covers
literally everything. I got the per-VIN custom manual from
Factory Manuals, but they do not have anything for the 2021 TB yet.