I had the same question and finally got an answer from ASUS:
They sent me a PDF in email when I raised a support ticket and refused thier initial answer that I could just download in from the support site, which of course you can't because they haven't published a BIOS guide there!
Dear Sir/Madam,
Thank you for your email.
Please find a general overview for series 600 intel boards attached.
It may have options described your motherboard does not have, as it is a manual for all of the mainboard in the Intel 600 chipset series.
PRIME /
ProArt /
TUF GAMING
Intel 600
Series
BIOS Manual
What they DID'T do is provide the direct link to the download!!
But you can search for the manual by name: (The bold text above).
I found a copy here:
https://www.manualowl.com/m/Asus/Pro-B660M-C-CSM/Manual/658989?page=1#manual
It doesn't cover everything and some of their settings explanations are rediculous and uninforming, but at least the core of it is there.