Stefan,
earlier versions of Photos, up to macOS 12, could open Aperture Libraries and create new Photos Libraries from them. The new Photos library has been created in the same folder as the original Aperture Library.
On these older system versions we could save the structure of the Aperture Library and maintain the albums and folders. And even migrate a referenced Aperture Library.
This is no longer possible on Ventura and . We an only import the photos and videos from the Aperture Library into an existing Photos Library and lose the structure.
When I try to import photos from a referenced library to Photos, the Import window remains empty, because there are no items inside the library package. You would have to consolidate your Aperture Libraries before importing them to Photos. There would be no point to do that as it would be easier to import the referenced images right away. This would at least preserve your folder structure as albums.
I second Yer_Man's advice to avoid a referenced library in Photos. It is not only ill supported, but also still buggy.
And it is not possible to use iCloud Photos, if the library contains reverenced items.
To use iCloud Photos would need to set up your photos Library as a managed library. Only one Photos Library can be the system photos Library and sync with iCloud Photos .
I migrated my Aperture Libraries to Photos while I could still use Aperture and photos together. The last system version to support this has been macOS 10.14 Mojave.