The offense simply scored and scored. We can lament FGs instead of TDs but had the Rams actually scored TDs instead of FGs, the score would have been well into the 50's and that hardly ever happens. But when you score almost every time you cross midfield, that's a pretty big blow to the opponent. That's part of the formula for how New England has been so dominant over the years.
Also: the O and D started going into ball control mode, and inserting 2nd string players about 5 minutes into the 4th quarter. The D used soft zones.
Basically, the Rams O scored often. And didn't have far to go. And clearly the Niners were bent on Gurley not beating them. And yet the Rams piled up close to 150 yards rushing. Goff didn't throw very many times because he didn't need to.
Let's give some credit to the Niners. They have generally played teams close and given very good effort. And, they have talent all over the place. They were depleted in the secondary and playing a back-up QB. But the back-up has been playing at a similar level to what Garropolo was (stastically) so I don't think QB was necessarily the biggest issue.
Overall, the Rams are simply a head and shoulders better than the Niners at this point. And it showed.