Android
You can add a snippet to your 1SE app for Android directly from your Google Photos library.
Head over to this article to learn more on how to use Google Photos.
iOS
It's not currently possible to select media from Google Photos directly within the 1SE app for iOS.
If you want to add media from the cloud via Google Photos, you'll need to download it locally first by going to Google Photos and select "Download." You can learn more about this here.
Once downloaded, this media will be available in your Library when selecting content in 1SE.
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You can still use Google Photos as a backup tool for media files in your device.
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