SDR is 24 bit and HDR is 30 bit. However there seems to be some brightness metadata required to specify how bright 1023 is. What baffles me is that there seem to be five or six different standards for this metadata.
None of HDR10, HDR10+, or DolbyVision ever answered the question of how to adapt for varying ambient brightness levels, either, which is again a very important question to answer for portable devices like phones, tablets, and laptops.
Those are extremely niche use cases. Most normal people watch their TikTok and YouTube on a perfectly calibrated wide gamut HDR OLED in their darkened color grading room.