What types of metadata should I use?

[Phil Harvey]

3. XMP is not a metadata standard, but a standard for how metadata is represented based on an XML.

No.  XMP is an extensible metadata standard.

1. I noticed that XMP is used to write the xmp namespace (-XMP-xmp:...), but I couldn't find a list of these tags from the xmp namespace.

I'm a bit surprised that you haven't found the ExifTool Tag Name Documentation yet.

- Phil

[Jom]

1. I noticed that XMP is used to write the xmp namespace (-XMP-xmp:...), but I couldn't find a list of these tags from the xmp namespace.

I'm a bit surprised that you haven't found the ExifTool Tag Name Documentation yet.

I was referring to official sources on the Adobe website.
You didn't come up with these tags yourself, you took them from the standard.
I just want to find the source.
But I looked at the documentation again
https://wwwimages2.adobe.com/content/dam/acom/en/devnet/xmp/pdfs/XMP%20SDK%20Release%20cc-2016-08/XMPSpecificationPart1.pdf
https://www.adobe.com/content/dam/acom/en/devnet/xmp/pdfs/XMPSDKReleasecc-2020/XMPSpecificationPart2.pdf
and it looks like you took information for your page (https://exiftool.org/TagNames/XMP.html) here.
Am I correct?

[Phil Harvey]

Yes