This is from the Athletic so you gotta pay, but they actually get into the weeds on how they grade players, I still think it's garbage but here is a snippet about Josh Allen:
If the grading system simply plucked from the stats, it wouldn’t be much of a system at all. Those who are trained in Pro Football Focus’ grading system get a 350-page manual on what to look for at each position. Players are assigned a score on each play up to +2 or as low as -2 in increments of 0.5. The scores are determined by attempting to isolate the quarterback’s play from those around him. A quarterback still gets credit for a good throw if a receiver drops a pass. Conversely, if a defensive back drops an interception, the quarterback is still penalized for the throw. While it’s subjective in some ways, it’s not guess work. They’re basing a grade on having watched thousands of plays across the league over many years.
Let’s use Allen as an example. He threw for 417 yards and four touchdowns. He pushed the ball down the field and made some challenging throws. If Stefon Diggs is running wide open, Diggs might get more of the credit for a big play in the passing game than Allen would in PFF’s grading system. On Sunday, Allen had two passes that were nearly intercepted. One would have been an easy pick six. Those dropped his grade slightly.
“We grade positives and negatives,” Palazzolo said. “He has more positively graded plays through two weeks than any other quarterback. It’s easy to remember those and forget the negatives. Because through two weeks, the negatives just haven’t come back to bite him.
“I don’t think people understand how we do it, because we’re just grading production and how you played. We’re not grading velocity on the ball necessarily or knee bend by a left tackle. So a lot of people can’t comprehend it.”