This may sound wired to some here, but in my experience the pixelcount has almost no effect on luminance noise.
You have to understand that zooming in 1:1 means that each pixel on your monitor represents 1 pixel of your picture.
This means that you zoom in farther into a given picture when using a hd monitor, compared to a 4k monitor.
This also means that, when viewing 1:1, you zoom in way way more into a 61mp picture than a 24mp picture.
Of couse there is more noise if you zoom in farther, thats just basic logic. That doesnt mean that the pictures of the camera are more noisy though. it just means that the 1:1 preview is more noisy and this is an important distinction.
The only real difference i could find, regarding noise, between high mp cameras and low mp cameras is that colour noise is a little bit better on low mp cameras.
Btw this is an 10.000 iso image of the sony a7r iv. looks absolutely fine and processed it looks even better.
In summary:
there is almost no perceivable noise performance difference between high and low megapixel cameras. Of course you can see more noise when zooming in 1:1 because you are zoomed in more. if you upscale a 24mp pic to 61mp in photoshop and then zoom 1:1 into that upscaled pic, you would see the same noise.
also: low mp has a little bit better colours at high isos -> images of low mp cameras will look a bit more colourful, while high mp low ligh pictures will look a bit more muddy (regarding the colours only). only a very slight difference though.