Last month Samsung announced the highest-res smartphone camera sensor ever, the mammoth 1/1.33" 108MP ISOCELL Bright HMX. Xiaomi will be the first company to use this in a shipping device, and so far a lot of rumors pointed towards the Mi Mix 4 being said device, when it launches presumably in October (that would be one year removed from its predecessor's announcement).
However, a new investigation by the enterprising folks over at XDA Developers has revealed that the Chinese company is in fact working on four different smartphones that will feature the 108MP rear camera. These are codenamed "tucana", "draco", "umi", and "cmi", and they've been revealed through code in MIUI's Mi Gallery app.
That app has added support for viewing 108MP photos in full resolution, but this is specifically meant for those four devices. Hence the inference that they would all be sporting the 108MP sensor - and be capable of shooting 108MP pictures, if you want to go through Manual mode and enable that. Not that you would, seeing as how the ISOCELL Bright HMX uses a quad Bayer filter so it's meant to shoot at 27MP with pixel binning giving an effective pixel size of 1.6μm.
The same source has in the past identified two Xiaomi phones that would employ pop-up selfie cameras, which turned out to be the Redmi K20/Mi 9T and Redmi K20 Pro/Mi 9T Pro. Of course the mere ability to view 108MP photos in the gallery app isn't a fool proof confirmation that the four Xiaomi handsets will sport the ISOCELL Bright HMX camera, but that logic does sort of make sense.
Think of it this way: if there's only one Xiaomi with the 108MP snapper in the pipeline, and the company wants to make sure that your friends can view the 108MP pics you snapped on that on their phones, then support for this feature wouldn't be limited to four specific devices.
Oddly enough, the source of this report says none of these four handsets is likely to be the Mi Mix 4. So either the rumors about it having a 108MP camera were wrong, or support for it has yet to be added to the Mi Gallery app.