Here is how to turn on iCloud Photo Library on macOS Catalina:
How to disable iCloud Photo Library on only one device
On a Mac: You can set up cloud sync from the Photos app. Once you have the app open, head to Photos > Preferences > iCloud. There, select the checkbox for iCloud Photo Library. You'll also find this setting under System Preferences > iCloud > Photos > Options.
How Do I Turn on iCloud Photo Library?
Apple gives users 5GB of free iCloud storage space for all possible purposes. However, all Mac owners may face the problem of duplicate photos filling up their storage space. Day after day, these duplicate and similar photos take up a lot of space on your cloud storage.
The only real difference here is that you'll need to move to iCloud Photos. If you recently created an Apple ID, you'll notice that My Photo Stream isn't an option. Only iCloud Photos gives you access to all your photos and videos from all your devices, superseding the abilities of My Photo Stream in most cases.
You can set up the same automatic syncing on the Mac, too. To do that, go to Apple menu > System Preferences > Apple ID > iCloud > check Photos. Then go to Photos app > Photos menu > Preferences > check iCloud Photos.
How to Backup Mac Photos to Your iCloud Photo Library
Your photos and videos are stored on your device in their original, high-resolution version.
Follow these simple steps to turn on iCloud and all your images will be automatically uploaded.
Recover photos and videos
iCloud Photos works with the Photos app to keep your photos and videos securely stored in iCloud, and up to date on your iPhone, iPad, iPod touch, Mac, Apple TV and iCloud.com.
Bottom line. For me, iCloud Photo Library is an excellent way to bring my images and video to all my devices. It boasts impressive sync capabilities, offline photo access, and easy management and backup, even for gigantic libraries. But it also has a few caveats that, while not deal-breakers for me, might be for others
In iCloud for Windows 11.1 or later New photos and videos you add to iCloud Photos are shown on your PC automatically. To download the file from iCloud when you view it, double-click its thumbnail. You can also keep your photos downloaded on your PC with these steps: Open File Explorer.