Um, you do realize that movies can't possibly show everything, right? By their very nature. The movie is compressed into two hours. The three-day trip from Tatooine to Alderaan was compressed (symbolically) into about five minutes. The several days of training and preparation before the Battle of Yavin were ommitted altogether. (You don't really think Dodonna managed to spot the thermal exhaust port after a three-second glimpse at the schema, did you?)
The movie included Red group (X-wings) and Blue group (Y-wings) (I'm pretty sure Gold group was only in RotJ; the Falcon and her escorts).
The novel (as I recall, it's been years since I read it) makes it pretty clear that Green (X-wings) and Yellow (Y-wings) groups cut across the Death Star's equator to draw fire, in various feints that prevented the Empire from realizing that the target was the thermal exhaust port.
It's like I always say: When the going gets tough, the tough . . . switch to artillery.