Combining Photo Libraries (Ventura)

To switch between Photos Libraries quit Photos, if it is running. Then double click the Photos Library you want to switch to in the Finder, and it should open in Photos. Create additional photo libraries in Photos on Mac – Apple Support (UK)

To combine to Libraries in macOS 13 Ventura, you have two options:

• You can import the photos from one library into the other library - just open the library you want to import, then use the command "File > Import" and select the library you want to import from. This will import all photos and videos from the selected library, that are not already in your c currently open library. Photos will import the original photos paired with edited versions, but the keywords will be omitted and the albums and folders will be missing as well. You will only import the media, not the structure of the library. Import photos from another library in Photos on Mac – Apple Support (UK)
• If you are using iCloud Photos, it would be better to merge the two libraries in iCloud. This way you will also import the keywords and the albums and folders. To merge the libraries, make the library you want to add your iCloud Photos Library and wait for the library to be uploaded to iCloud. Once this is complete, make your main library again your iCloud Photos Library. Then the merged library will download into your main library. This can take several days, if your libraries are large. Use iCloud Photos to store photos in iCloud – Apple Support (UK)

I see, OT already explained the first part.

Just double click on the library you'd like to open. Or launch Photos with the Option key held down and select the library you wish to open.

Be aware that merging library with Photos V 8.0 does not merge the albums or folders with albums. It just merges the photos and metadata.

The paid version of PowerPhotos will merge libraries and albums, photos (both edited and original), metadata and projects. However, projects are brought over as albums because themes have changed between versions of Photos.

If you've done a lot of organizational effort in the older library you may want to consider PowerPhotos.

After you import the photos from one library into another library, you can dismount the external drive with the imported library.

There is no toggle switch in Photos to switch quickly between libraries.

You select the library you want to switch to in the Finder and double click it.

• If Photos is currently open with a different library, it will ask you, if you want to switch to the other library.
• If Photos is not running, it will open the clicked library instantly, without any questions.

Hold down the Option+Command keys and double click on that library to open and repair it.