Rich's Tongfamily (Richtong.net add-on)

Well, I’ve had lots of posts about this over the years and it is a confusing topic. But, basically here is what you need:

1. The Monitor. You need one that processes HDMI 2.1 which allows 4K 120Hz and 4:4:4 that is no color compression. The LG B9 and all later models (this is five years ago!) do that. The main issue is getting it all to work
2. The Cables. You need an HDMI 2.1 capable cable that supports 48Gbps so that it can all get down to your machine
3. The Adapter. With a MacBook Pro, you have a choice of the Thunderbolt 4 outputs which are a good 40Gbps, so then you need a Thunderbolt 4 to HDMI cable that supports Displayport 1.4 at least as a protocol. If you are using DisplayPort, then you need a cable that is at least capable of going from DisplayPort 1.4 to HDMI 2.1.
4. The Dock. If you have a docking station in between like me, then you need a CalDigit Thunderbolt 4 for instance that supports DisplayPort 1.4. The older CalDigit Thunderbolt 3 dock supports DisplayPort 1.2 so can only go to 4K 60 Hz and no HDR.
5. What does the Mac see? If you get this going, which you can tell by going to Settings > Displays and looking at the monitor output, it should show a Refresh Rate of 120 or 60Hz and then if you have HDR, you should see High Dynamic Range as a toggle. Note that in the interface there is no way to tell what color compression you are using. The easiest way to get this is to see what Netflix.com is producing with an HDR video. With the limitation of HDMI 2.0b maximum, you can get 4:2:0 typically.
6. Applications. This is also complicated, but basically, you can set Safari to do HDR output and then you can use IINA for HDR movies that you have on your machine. Also Final Cut Pro supports HDR output directly. For streaming, you can use the Apple TV application or if you set Safari Developer preferences and Netflix on Safari, but it’s somewhat tricky to get Dolby Atmos out of it. Note that these preferences seem to have changed, but setting the Settings > Battery not to convert to SDR on the battery is still there, but now it seems the VP9 profile 2 codec is enabled.

Why go through this brain damage? Need for more ports

It’s because I need more Thunderbolt 4 ports. CalDigit Thunderbolt 4 dock, I have attached it to a Razer Thunderbolt 4 dock because I don’t have enough USB ports. Yes, that’s true and I have another Thunderbolt 4 SSD. So when I was using the very good Sabrent Thunderbolt to HDMI 2.1 adapter, I needed three ports. The solution is to use the DisplayPort hardware, but now I need a new cable.

Net, net I have the older CalDigit Thunderbolt 1.2 to HDMI 2.0 cable so I need the newer DisplayPort 2.0 to HDMI 2.1 cable and will report back. This is $25 from Amazon.

Note that there was a hack before Sonoma 14. 1 that lets you use a hacked Cable Matters chipset to get 120Hz HDR 4K, but this seems broken now. The issue is a deep but frustrating one in that the output clock internally is set to a maximum of HDMI 2.0a even though the hardware can support HDMI 2.1