Yes, Nokia 8.1 display works as a flash under low-light conditions.
Nokia 8.1 has LPDDR4X RAM.
Yes, Nokia 8.1 does have the HDR10 support and you can watch 4K videos on Nokia 8.1. You can record 4K videos also.
Yes, Nokia 8.1 does support Qualcomm Quick Charge. It also comes with 18W fast charger.