Most users will have a single library, held by default in the Pictures folder in their Home folder, that is designated the systems library and what is shared across all devices via the Cloud. There is no compulsion to use the cloud and some people prefer to sync their devices by WiFi, Cable or even AirDrop.
On the Mac it is possible to have multiple libraries only one of which will be linked to the Cloud. To switch libraries you can double click on the specific library or Option Click on the Dock Photos icon to open a window listing available libraries or to create a new library. Note that whatever the last library used will stay the default library and to switch back to say the system library you have to deliberately select it.
You can import photos from one library to another all from within the Photos app itself. Open the library into which you wish to import photos, select File Import and click on the library from which you wish to import. It is opened in the Photos windows giving you the option to import all new or just select photos to import.
David also wanted to see if you can export one or a few photos from one library and import to another with out loss of detail such as a Live Photo. When he exported a Live Photo it was stored as two files a .jpeg and .mov but when imported into another library showed as a single file and was a prefect Live Photo.
David showed how to split a library by simply duplicating and renaming one and then deleting photos, as required, in each library from within the Photos App
Only the system library is connected to the Cloud and is the only library accessible to all other devices. So, as far as David is aware, only the Mac can handle additional libraries.
Whilst Apple does not recommend any third part apps accessing Photo libraries, the App Store still offers apps to remove duplicates. David gave a short presentation on Photo Sweeper which he finds works well and is available from the App Store for under £9.00. Making a backup of a library before messing about using a third party is strongly recommended.