Move over, Galaxy M31: it looks like your title of "battery monster" will go to a newer phone soon. Samsung has apparently made a new mobile power-pack with a rated capacity of no less than 6800mAh. It has leaked out thanks to a few official certifications for this component from Korean and Chinese regulators.
https://www.notebookcheck.net/New-leaks-hint-at-Samsung-s-next-big-thing-in-smartphone-batteries.479630.0.html
https://www.youtube.com/watch?v=5e6vcl9Jwb4
Ningde is most notable for manufacturing batteries for EVs, so it's unlikely that this has some sort of special technology, likely just trading thickness for capacity, not that there's anything wrong with that, most flagships need that.