Mannion has a bigger arm in terms of distance, but not really in terms of accuracy, which is largely due to Goff's superior ball velocity and overall mechanics. Riley runs a pro-style offense, it doesn't rely on short passes to stretch out the defense, he also adapted from spread offenses in terms of really simple play calling (not a lot of verbiage).
Goff's pocket presence was always one of his strengths, here's a good breakdown of him displaying that in college, and there were plenty of times in the NFL you could see him sense the pressure and roll away or avoid a sack to extend a play. Now there were also plenty of times where he was too slow to act, held the ball too long and such, but that's part of being a rookie. Mannion does that as well still.
Mannion throws well from a clean pocket (he was part of a pro style offense, so of course he can throw from the pocket) but he lacks the agility to slide and avoid pressure within the pocket, and if protection breaks down he really struggles, vs Goff has demonstrated the ability to stay alive and complete the pass. A lot of the issues that Mannion have stem from his footwork, he's very heavy footed and that causes issues, including getting the most from his arm, which is why most NFL scouts knocked his arm despite the power he can get. It's also why he doesn't have as much velocity to his throws that Goff does, so Goff's ball gets there faster, which is very important for throwing into tight windows.
Mannion also has the same sized hand as Goff, despite being 2 inches taller, which was a knock but not a big issue to me.
Most of Mannion's issues can be fixed as they come from poor mechanics, footwork and throwing motion specifically. While I'm sure the coaching staff will work on that (because why wouldn't they?) it's not as if they're going to spend extra time when they have a QB on the roster who already has the mechanics they need. That's why McVay said that Goff is the starter, that's why McVay and Goff were watching film together for hours, breaking down plays together, during the interview process. It wasn't McVay and the QB's, it was McVay and Goff.