You linked the final autopsy. The document submitted when they arrested chauvin was, I believe, called the "Charging Document". It included the description of floyd complaining about not being able to breath to the original officer who handcuffed him before chauvin was on the scene. The info must have come from body cam reviews. It also included the preliminary autopsy before the toxicology results were in.

There are a bunch of studies about fatal fentanyl levels. Here is one that says:

"Postmortem levels of fentanyl...range widely from 0.75 to 113.00 ng/mL, with a mean of 9.96 ng/mL"

https://www.google.com/url?sa=t&source=web&rct=j&url=https://ndews.umd.edu/sites/ndews.umd.edu/files/ndews-hotspot-unintentional-fentanyl-overdoses-in-new-hampshire-final-09-11-17.pdf&ved=2ahUKEwjgnfTb3o7qAhVWaM0KHRl0C00QFjAEegQIBxAB&usg=AOvVaw3xpBPC7rnNMpZnfyirgYcq