Download photo from icloud to camera roll

When you open a photo on Photos to view it enlarged (zoom in on the photo) it will be downloaded from iCloud and be accessible for other applications.

If you want to have all iCloud photos and videos to be stored locally on the iPhone as well, disable "Settings > Your Name > iCloud > Photos > Optimise ..." and switch this to "Download and keep originals". This will only work, however, if you do not have too many photos and videos in iCloud. Yoe need enogh free storage on the iPhone to keep all photos stored locally.