Well your main question was whether there was a 64MP sensor there
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You'd have to ask Samsung why they made certain decisions, but the first thing to remember is that this isn't a 64MP telephoto camera: it's a 64MP wide-angle camera (almost as wide as the primary) and that the zoom comes from cropping and software interpolation (i.e. digital zooming). The difference is that the 64MP gives you a bit more scope for cropping before you start to rely heavily on software for the zoom (there is no real optical zoom on the S20(+), only on the Ultra).
So as for what's happening, I don't have one of these cameras so am just working from what I can read plus a bit of educated guesswork. But here goes:
The first thing to note is that you won't get 2MP from a 6x crop of as 12MP image, nor will you get 10MP from a 6x crop of a 64MP image. When people talk about zoom ratios they refer to the angle of view, i.e. the linear magnification, while 12/64MP refers to the area. So a 6x zoom by cropping a 12MP image would give you a 0.3 MP image (VGA resolution). So digital zoom isn't simply cropping: they crop and then they do a software interpolation to scale that back up to a higher resolution. Try taking a picture, zooming by a factor of 2 then taking another, then look at the properties of the images: you'll find that they both say they have the same number of pixels - that's using software to "scale up" the resolution back to what you set it to. Of course you don't get extra information this way, so the real image quality is always inferior with digital zoom compared to a true optical zoom.
So my guess is that if you select "12 MP 6x zoom" it will produce a 12MP image using software zooming (about 4000x3000 pixels). Now set it to 64MP and tell it to take a 6x zoom image: what resolution does it say the image has? Does it give you a 64MP zoomed image, a 12MP one, or something inbetween (for small zoom factors)? My guess is that it will rescale to 64MP, to be consistent with what the setting said (you asked for 64MP so the camera gives you it), but I'd be curious to know whether I'm right. If it does something different then it changes what I speculate below.
Then it gets more speculative: you say that you get 30x zoom in the 12 MP setting. What resolution are the 30x zoomed images? My guess is 12MP. Now an actual 30x digital zoom would be utterly worthless: a 30x linear crop contains 1/900 of the original pixels, so scaling that up to the original resolution is going to look like crap. So my theory is that the "30x zoom 12MP" is actually using the 64MP sensor, just rescaling back to 12MP rather than to 64MP (it will still look like crap, but not as bad as doing it with the 12MP sensor would).
As I say, I don't have one of these phones to test, so I may be wrong. But my guess is that it will do something like this:
* When you select 12MP or 64MP you are not so much choosing which sensor you use but what resolution the image is.
* If you set 12MP mode and start zooming, at some point it will switch from using the 12MP sensor to using the 64MP sensor, but it will always rescale the zoomed image to 12MP. At what point it will switch I don't know (6x would be an obvious guess, given where the 64MP mode ends, but it might happen earlier - it might even happen the moment you start to zoom).
* If you set 64MP mode it will use a digital zoom up to 6x, rescaling the image to 64MP. Above that it will stop because the image quality loss is too large for it to be worth rescaling up to 64MP, and so if you want a > 6x zoom you have to choose lower resolution (i.e. 12MP, even if that actually uses the 64MP sensor for part of the range).
But as I say, that's only a guess. It could even be that they just use digital zoom on the 12MP sensor all the way to 30x and the purpose of the 64MP sensor is to give higher quality zoom images, and they cut that at 6x because anything more than that is losing too much quality. But given how outlandish a 30x digital zoom is my guess is that they actually use the 64MP sensor for that then rescale to a lower resolution - at least if you asked me to implement a 30x zoom with that hardware, that is the way I'd do it.
