I do think a well-known simmer with many followers will have an advantage because more people are notified when they upload.
For regular user uploads, I think the vast majority of people download based on the main screenshot.When you're browsing the main gallery, there are thousands of builds coming in at a constant flow, so your screenshot has to stand out. You can change how the screenshot looks by changing the time of day, and even by going into live mode and having different seasons. Even the angle of the camera matters, because I once placed a wedding venue on a lot that blocked the entire building with a pine tree. If you change the location of the lot, the background will change and can sometimes accentuate your build. If your main screenshot looks really good, people are going to click and look at the other screenshots. Otherwise, it'll get skimmed over and people won't even click to see how the interior is built. This has happened to me once before on my Pavilion lot. I think it took me months to actually finish, but because the screenshot was to the side instead of the front of the build, much less people checked it out. I think it had about 40 downloads but of those downloads about 75% of the people Favorited it when they a actually looked at the interior.
I also think people download based on the type of venue and hashtag searches. One of my castle builds has hundreds of downloads, even though it is just a shell. If you've build something someone else can't build or don't have the time/patience to build, it'll get more downloads.
From my own uploads I've also noticed that public venues (such as restaurants, bars, gyms) get downloaded more than my residential builds. I think it's because public venues are lots that more than one sim household can enjoy.