1) There is no formula. Graders grade individual plays watching the all-22.
2) Other graders then check the grade making sure it is accurate.
3) Each play is graded -2 to 2 in .5 increments, where 0 is expectation, -2 is catastrophic mistake, and 2 is absolutely perfect.

So many reasons why the passer rating is higher, but PFF grading is lower.
1. The WR were so open that a bad throw doesn't matter. The passer rating sees 70 yards and a touchdown, while PFF sees a ball that was expected to be made.
2. The WR makes a crazy catch. The passer rating sees 30 yards, while PFF sees the ball was overthrown.
3. An incompletion: The passer rating sees 0/1, while PFF sees the QB missed a wide open receiver.
4. A check down: The passer rating sees a 5 yard completion to a RB, while PFF sees the QB missed 2 wide open receivers, which would've given 25 and 15 yards respectively.
5. The list
6. goes on
7. and on...

4) The +/- score is then converted to a 0-100 score. This means that how much you play does not factor into your score. Cam Rising's grade was not impacted by only playing one half. Edited for clarity: normalized by number of plays. See Luke To'omalatai being BYU's fourth highest graded player, but only playing 14 snaps.