For smaller batches, you can select multiple photos within the Google Photos app and use the download or share function, though this is not practical for a large library.
To download all your photos from Google Photos, the most reliable method is to use Google Takeout (takeout.google.com) to create a downloadable ZIP archive of your data, then transfer that ZIP file to your phone and extract it.
You can select multiples to download. Just long press one and then check any others you want to include. Unfortunately, Google backup does not keep your album format or storage location or your "favorites" tags. All photos will be downloaded to the DCIM folder and show up in Gallery in your "camera" album. You will have to recreate your albums in Gallery. If you have many photos, you don't have to re-order them all at once, you can do it during periods where you have some time to kill. The important thing is that you still have your photos.
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