Well, and yeah, all answers are wrong here. HDR is only the new transfer function, PQ or HLG. So that means you need to look into that, PQ is SMPTE 2084 and HLG is ARIB STD-B67.
Yes, 8 bit file tagged as PQ will be played as HDR.
Yes, 10 bit files tagged as BT.2020 transfer are SDR, because BT.2020 transfer is the same as BT.709 and BT.601. Moreover Rec. BT.2020 is SDR only spec., HDR was defined in BT.2100. Master files of Hollywood used 10 bit in SDR for a long time.
Yes, you need to color manage BT.2020 primaries to your primaries. Just as before PAL primaries were supposed to be color managed to what display primaries are.
Yes, it is 0.0001 to 10000 nits, not 0.001 to 4000 in PQ. Yes, 65 Y' is actually 0.00005 nits and 64 Y' is 0.000000 nits. Black body.
Yes, BT.2390 EETF dynamical tone mapper is mandatory unless the master display is full 10000 nits.
Yes, almost all movies are inside P3-D65 inside BT.2020, only Planet Earth 2 and Matrix are not and some others. That changed in 2022, since Netflix allowed to use BT.2020 primaries for their JPEG2000 masters.
Yes, film cameras supported HDR back in 1970s and they are nowadays converted to PQ and image format .pcd (Photo CD) supported HDR and WCG.