At what point can you see the difference in a higher MP camera?

I used to have the original Sony A7R, 36 MP, had very nice Zeiss glass. I went back to Canon, I had the 5D III before, now I have the 6D.

Rather I'm on my retina display or desktop display, I cannot tell that one is more high resolution than another. At what point does that happen? I'm not speaking about prints.

I'll give you an example: Even though my monitor is retina and not 4k, when I record videos on my Sony video camera in 4k, or watch other videos in 4k, it clearly looks better than 1080p, even though I'm watching it in 1080p. This is because there are 3 times as many pixels in each box as 1080p, so 1080p 8 bit 422 looks more like 10 bit with 4k. Its sharper, less noise, etc.

Now that's 4k which is 8 MP playing in 1080p which is about 2 MP on a computer monitor that is about 2k.

So, at what point, let's say on a 4k display, an 8 MP display, does one notice a difference? Without zooming in. Can one tell the difference between 20 and 42?

Obviously, cropping, zooming in 1:1 on lightroom, etc, once can see more resolution.

But I look at my Canon photos on my computer screen and my A7R photos. Both look amazing, I prefer the Canon "filmic" type warm look it has. But I cannot tell a difference.

I'm wondering at what point can you?