A test would be very welcome examining the resolution loss from 1.06x to 2.5x and 2.5x to 5.9x, both subject-normalized by, for example using the same imatest chart to fill the frame at each setting, in order to determine the difference between "hybrid" zoom and digital zoom, if there is one.

The S20 has no optical zoom. The lens on the "telephoto" camera has same field of view as the main camera. It achieves ~2.5 "zoom" by heavily cropping the sensor.
So you essentially get a 12mp image that was heavily cropped from the 64mp sensor. The sensor doesn't use Pixel Binning in this mode as it needs all pixels to produce a 12mp crop, so you get effectively a tiny sensor and tiny pixels.

NO, it is NOT 2.5X optical zoom. Both S20 and S20+ have ONLY 1.05X optical zoom, and the rest is purely digital. Here is how it works.

If you look at the official spec sheet you see they have a 64MP telephoto lens with 76 degrees of FoV. This roughly translates into 28mm equivalent FL, which in turn shows this 'telephoto' camera features a WIDE ANGLE LENS, just a bit narrower than the main camera's.

What does this indicate? Basically, S20 and S20+ have two main cameras with different resolution, and switch to the higher resolution 64MP camera from the regular 12MP camera when a user starts zooming in. This is NOT how optical zoom is supposed to work on a phone.

What a disappointing advertising strategy from Samsung, I can easily imagine average consumer falling into their misleading descriptions. I'm not impressed.