Adding columns to SharePoint Admin Center - Active Sites

ChrisSouthgate  from SharePoint Admin Center you can select a site, but to directly add columns (or metadata properties) to sites, you'd need to work with the Site Collection settings. In SPO, columns are generally added using Content Types. For site-wide columns, need to enable Content Types via Site Settings.

To add columns across all sites:

1. Enable Content Types at Site Collection Level:                                                                                          In Site Settings, go to Site collection features and enable Content Type Syndication Hub.
2. Create/Modify Content Types:                                                                                                                    In Site Settings, under Site Content Types, create or modify a content type and add your columns.
3. Publish and Apply Content Types:                                                                                                              Use the Content Type Hub to push content types across site collections, or manually add them to individual sites under Site Settings > Site Content Types.
4. Apply to Libraries:
   Enable content type management in Library Settings and add the content type to libraries.