How do I select photo libraries on an iPhone running the latest iOS?
I Have eight photo libraries setup on my iMac.
How do I select these individual libraries on my iPhone?
You can't
You can only sync one library with your phone (the system library on your mac).
You can change which one is the system library (in preferences) but that will then require a complete new sync - and if you are using iCloud - will merge the new system library with what is currently in iCloud (the old system library)