Reducing crushed black levels on XDR displays
The XDR displays have very good black levels, close to OLED. When watching SDR content, these displays can crush the darker content in an image. The following page shows black levels to test. The first 6 on XDR with the default profile look indistinguishable:
The default color saturation is also high.
In System Prefs > Display, it allows setting up a custom profile under presets > Customize Presets but this panel shows basic options and setting up a new preset locks the display brightness.
There is another way to setup a new display profile as described here:
/System/Library/ColorSync/Calibrators/Display Calibrator
Hold the option-key when launching this to enable expert mode. Go through the steps. Many can be left at default.
Disable True Tone in System Prefs > Display before adjusting.
It's good to have an SDR video open with a dark scene when adjusting.
Here for example at 0:13, look at the lapel on the jacket below the neck. On the XDR display, it's not visible:
The following movie has a lot of dark scenes with dark clothing like at 0:26:
To improve the visibility, the following settings can be used (adjust more or less according to preference):
Set screen brightness around 50% during test.
Increase the white point to 6800 so white appears more blue than yellow.
Change gamma to 1.9.
Save it to file.
With this profile, it reduces color saturation (more natural skin tones) and shows more details in dark scenes. Check the black level site above and now more black squares should be distinguishable.
To switch between profiles, use Utilities > ColorSync Utility > Displays > Color LCD and switch the current profile between factory and the adjusted one.
Here are some differences, brightened to show more clearly. The creases and lapel on the jacket on the default XDR profile blend together into solid black but are visible on the right. The red lips and skin tones are heavily saturated; on the right, they are more natural tones.

In the following scene, there are a few differences. To the right of the white shirt there is a highlight missing in the left image. A highlight missing at the top of the character's right shoulder. The steps to the left are brighter in the right image. The details in the neck area next to the collar are more visible in the right image. The green and red highlights on the face are more visible.

It's worth adjusting the calibration settings as most content is SDR and it can lose some detail. This calibration still has great black levels with less crushing.
The following HDR video is also worth testing against to see the difference in the profiles.
https://www.youtube.com/watch?v=njX2bu-_Vw4
Here are some differences, brightened to show more clearly. The creases and lapel on the jacket on the default XDR profile blend together into solid black but are visible on the right. The red lips and skin tones are heavily saturated; on the right, they are more natural tones.

In the following scene, there are a few differences. To the right of the white shirt there is a highlight missing in the left image. A highlight missing at the top of the character's right shoulder. The steps to the left are brighter in the right image. The details in the neck area next to the collar are more visible in the right image. The green and red highlights on the face are more visible.

It's worth adjusting the calibration settings as most content is SDR and it can lose some detail. This calibration still has great black levels with less crushing.
The following HDR video is also worth testing against to see the difference in the profiles.
https://www.youtube.com/watch?v=njX2bu-_Vw4
Comments
Some of the visual quality difference here is due to Youtube compression. Brighter images show the compression noise. The black levels being crushed hides the noise but also the detail. On higher quality video compression, it just hides the details.
One of the most obvious differences in the images is the steps in the 2nd image. The top step is much less visible on the left side.
You have to view these images on an XDR display to see the difference. The images will look fine on an IPS display.