Hi,
Thanks for posting in our community.
Based on the description, it seems like your concern is why the columns are not showing under the Details Pain for a selected folder. If my understanding is correct, kindly check the following details.
Generally, the content in the Library is associated with the content types and the files in the Document library content type is different from the content type of the Folder. For the folders in the Document Library, the default Content type is Folder and for the files, the default content type is Document. So, the columns created in the Document Library are associated with the Document Content type where it's not associated with Folder Content-type.
This is the possible reason why the columns created in the Document Library don't show in the Details Pane.
Besides, we do understand your demand to add some similar properties for folders in a document library, but we may be sorry to convey that currently there is no out-of-box way to add such a customized property for folders, where we need to create a new Folder Content-Type and create new columns to be associated with the content type, then add this content type to the Document Library and create new folders from the content type. Kindly submit your feedback to the SharePoint building team in SharePoint · Feedback Community (microsoft.com)
Your understanding is highly appreciated and hope a nice day for you!!
Best Regards,
Mia