Hi @SHUO .H ,
Great to know that and thanks for sharing the update here.
By the way, since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others.". and according to the scenario introduced here: Answering your own questions on Microsoft Q&A, I would make a brief summary of this thread:
[Regarding the Content Type Gallery in SharePoint Admin Center]
Issue Symptom:
I create a custom Content Type with specific columns in Content Type Gallery in SharePoint Admin Center. What I'm curious about is where the COLUMNS are stored??
Current status:
I have found out the Content Type and Site columns live in the following URL: domainname.sharepoint.com/sites/contenttypehub URL and it was a hidden site collection that we could only access /sites/contenttypehub at the end of tenant URL.
You could click the "Accept Answer" button for this summary to close this thread, and this can make it easier for other community member's to see the useful information when reading this thread. Thanks for your understanding!