External Drive System Photo Library

I have 2 apple devices, my MacBookPro running Mac OS Mojave and my iPhone 8 running IOS 13.3.1. I have been using an Seagate 2TB external drive for my Macs system photo library for some years now and have 13777 items in this library. I have created many "albums". I recently decided to turned on iCloud photos to keep my photos in the cloud. My iCloud Photos has synced all my iPhone photo items to the iCloud photo library but not the photos from my mac's external drive containing my system photo library. How do I get my mac's external drive containing my system photo library into iCloud photo?