Probably not much difference remaining.
50.2 megapixels is 8712x5808 pixels.
20.2 megapixels is 5505x3670 pixels.
(based on your megapixel numbers, I did not look up the sensors).
1920x1080 is 2.07 megapixels, so substantially less detail is possible.
A pixel is the smallest dot that can be reproduced, the smallest detail that can be reproduced.
8712 down to 1920 pixels is 4.5 to 1, or 1 pixel remains for every 4.5 pixels of width.
5505 down to 1920 pixels is 2.87 to 1, or 1 pixel remains for ever 2.87 pixels of width, which was itself only one pixel for every 1.58 pixels in the 50mp image.
This of course assumes the lens and scene did in fact transfer the maximum possible detail. Not always the case. Situations vary.
Expectations of 50 mp or 20 mp of detail in 2 mp of image are not very realistic. :) It might be a real good 2 mp image, but 2 mp cannot be better than that, detail wise.
In these two cases, resampled resolution will be 1/4.5 or 1/2.87 of the theoretical maximum original detail originally possible. In any case, it cannot be more detail than 1920 pixels can show.