Besides working as a viewfinder, the Z20's secondary screen can also double as an always-on display, showing you your notifications on the back of the phone. If you want to save battery life, Nubia suggests you can use it as your primary display in a pinch. Similarly, it's possible to use the display for multi-tasking by putting two separate apps on each screen -- though constantly flipping your phone seems like it would get tedious quickly. It's also worth mentioning that the Z20's secondary display is both less bright and not as dense as its main counterpart, so whether you'll actually want to use it frequently is hard to say.
Otherwise, the Z20 is about what you expect from a high-end Android phone in 2019. It features Qualcomm's new Snapdragon 855 Plus processor, which has only made its way to a handful of phones this year, including the OnePlus 7T and ASUS ROG Phone 2. Additionally, it includes 8GB of RAM and 128GB of internal storage. Powering the phone is a 4,000mAh battery. With a 27W power adapter, Nubia claims you can charge the battery to full in under 100 minutes. As for the main display, it measures in at 6.42 inches and features a 2,340 x 1,080 resolution.
For camera hardware, the Z20 comes with a primary 48-megapixel camera with an f/1.7 aperture lens. It also includes a 16-megapixel wide-angle camera and an 8-megapixel telephoto lens with three times optical zoom. In addition to the usual video capabilities, the camera can capture 8K video -- though only at 15 frames per second, which means you probably won't end up using it a lot since no one wants to watch a slideshow posing as a video.
While you can't buy it through a US carrier, the Z20 is available to purchase directly from Nubia's website for $529.