If you reduce the size of a 300ppi 10"x10" photo of a dog to a 300ppi 5"x5", you will lose 3/4 of the information (pixels) in the photo. In other words, you have downsampled the photo by removing pixels.
On the other hand, if you reduce the size of a 300ppi 10"x10" photo to 5"x5" photo without downsampling, the pixel count remains the same. You have squeezed the same number of pixels into a smaller area. In doing so, the photo increases from 300 pixels per inch (ppi) to 600 ppi because you’ve packed the same number of pixels closer together.
It’s only possible to downsample (or upsample) raster images since they’re composed of pixels. It’s not possible to downsample (or upsample) vector images because the information in a vector file is not composed of pixels.
Honestly, I’m surprised you didn’t know this. Not downsampling “out of precaution” doesn’t make sense vectors are never touched by those settings, they only affect raster images. For large-format work like wraps, oversampling just bloats the file and slows the RIP without making the print any sharper. The real precaution is just making sure your images are at a sensible resolution.
Here’s the simple maths: a 72 ppi image at 100 inches scales down to 300 ppi at 24 inches. Shrink it, resolution goes up; enlarge it, resolution goes down. That’s all there is to it.
For context:
Books and art prints: ~300 ppi (it’s the standard but actually incorrect)
Newspapers: ~120–150 ppi
Wraps: ~100–150 ppi
Billboards: ~30–50 ppi
So the trick isn’t avoiding downsampling “just in case” it’s understanding the output, the intent, and the scale you’re working at. Once you’ve got that, the settings make sense.