Pixel-level noise matters when you consider one or another sensor for your purposes.
It's also easy to compare sensors of similar megapixel count, e.g. R5, A7RIII, 5DsR, Z7.
When you compare sensors with different Mp count, such as R5 and R6, you take only one metric (e.g. dynamic range or noise) and deliberately downsample the R5's images thus losing 55% of information. 55%! As a result you're not comparing the sensors, you're comparing only the noise in normalised images. This comparison ignores the resolution and is only valid for cases where you downsample 45Mp to 20Mp. If you want to use all 45Mp from the R5, or downsample it to 30Mp, this comparison becomes totally useless.
No! You normalize, it doesn’t matter if you normalize up or down, the noise in normalized images is a direct comparison. I don’t understand your disconnect here, you don’t throw anything away and you don’t make stuff up, that isn’t what normalizing is doing, it is putting things on an even footing.
But you don’t need to resample up or down, just look at stuff at the same size not the same ratio (percentage). Full screen, thumbnail, print size it doesn’t matter, if the lower pixel image goes above 100% to view the same output size as the larger it is irrelevant, it is still a normalized comparison.
Online tools have to resample the higher resolution to a lower one so the display works, to make it simple they resample everything, high and low resolution sensors, to a level no camera will realistically go below. The 20mp camera and the 45mp camera are both downsampled to a common 8mp, but in your own home you don’t need to do that,
just look at both on screen so any subject or detail in both files is displayed on your screen the same size. That is normalizing, no resample, no throwing away anything no making stuff up.