Photos stored in iCloud should appear in-line with the rest of your downloaded photos in the Photos app.
If you have photos stored in iCloud Drive, you can access them with the Files app on your iPhone. Using Files, you can navigate the folders in iCloud Drive and other cloud storage providers, as well as files stored on your iPhone. Apps have to integrate with Files, and I'm not sure that apps by Facebook, like Facebook and WhatsApp have this integrated.
You can download any file, including photos, by tapping the download button in the top right corner of the photo's thumbnail in Icon view or by tapping the download button to the right of the file in List view.

To share these photos in other apps, hold down on the photo that you want to share, then tap Share from the context menu. 
From here, you can either share directly to the app you choose, or save the photo to your camera roll in the Photos app, then share the photo from in the app by accessing your camera roll.