Here's some prep things to do to make sure that your Blu-ray HDR experience will be a great one . . . . and a couple of answers to your questions.
NOTE: Great news! We have almost matching equipment, so there will be no guessing here. The only difference is that I have the M70-D3. They are pretty much the same TV except for the panel size.
1) Make sure that both the Pioneer and the Samsung have the latest greatest firmware updates available. If both are connected to your home network, and therefore the internet, you can update over the internet instead of using a USB drive. The Samsung is pretty good about keeping itself up to date automatically, but you have to tell the Pioneer to do it, or it won't. Let me know if you need guidance on running an update on the Pioneer and I will type something out. When the VSX-90 first came out, it did not support HDR passthrough. There have been around 4 firmware updates since it first shipped to the stores.
2) Make sure that you have a UHD/HDR Blu-ray movie. Not sure what kind of movies you like, but there are more and more titles coming out every week. "Life of Pi" is a fantastic reference example of UHD/HDR at its best.
3) HDR ( High Dynamic Range ) is special instruction coding for the TV that is contained in the signals of UHD Blu-rays or streaming services such as Netflix. These signals will put the TV into a specific HDR mode depending on the signal type. UHD/HDR Blu-rays played on the K8500 will playback HDR-10 signals. Some Netflix titles will playback Dolby Vision signals. The TV will automatically detect and switch to the appropriate mode. Regular or non-HDR signals are known as SDR. ( Standard Dynamic Range ). That includes HD Blu-rays, AppleTV, cable/satellite, etc.
4) Use the "Calibrated" picture mode on your TV and set the backlight to 40. Later, once you get things up and running, you might want to consider using my calibration settings which were done with professional calibration gear. Most of the members who have tried my settings love the picture quality - especially those with 65" and 70" panels.
5) Make sure that the Samsung is plugged into HDMI ports 1-3 on the Pioneer. The one labeled BD will do the trick. The Samsung needs the input to be HDMI 2.0a and HDCP 2.2 compliant for UHD/HDR.
Answers: I route everything through the Pioneer to HDMI 1 on the TV. I have DirecTV 4K, AppleTV, and the K8500. HDMI 5 does not support HDR. If you have a gaming console such as an xbox, you would plug that into HDMI 5. HDMI 5 has a low lag connection - crucial for massive gaming.
I think that should keep you busy for a couple of minutes. Let us know how it goes. Once the basics are up and going, we can get into more advanced fun.