What EOTF(s) do MacOS and Windows use for HDR content?

It seems to vary (or maybe just scale) under different conditions such as:

• With MacOS, you can use the HDR video preset to get the PQ EOTF. That's pretty clear.
• When you change brightness in MacOS in the default preset, such as Pro Display XDR (P3-1600 nits), what EOTF is used? Does it vary across the brightness range as the SDR / HDR limits are changed? Apple documentation references a system gamma of 1.961 (at least for internal displays).
• With MacOS, you can create custom presets using an SDR EOTF based on sRGB, BT1886, or a custom gamma. What EOTF is used for the HDR range above that?
• Windows HDR brightness slider for external monitors seems to potentially alter the EOTF

Thanks for any insight into this!

Target EOTFs are always as defined by the colour space standard.
They are effectively set within the monitor when calibrated.

What 'other' non-standard variations any given OS does via its own 'colour management' is really an unknown, but is a perfect example as to why a dedicated video card is critical for any colour grading work.

To define the effective EOTF on any display, you can obviously use ColourSpace to profile the via the OS.

Steve