Open any photo at Google Photos.
Tap the 3-dot menu or swipe upwards.
Choose Download. If the photo is already on your device, the option will not appear.
On my Android device the photo is saved to DCIM/Restored and in Google Photos shown in Camera.
When you used "Free up space" all photos are still in the Photos tab of Google Photos, but the folder structure is lost.
When photos were uploaded with setting "Storage saver" (or former "High Quality") the original is also lost.
On iPhone or iPad
On iOS you can select multiple photos > use the Share button > Save to device.
See also https://support.google.com/photos/thread/86166997
You can also use iTunes.
On Android (and some also on iOS)
New: in April 2025 the option to download multiple photos on Android is announced.
See https://www.androidauthority.com/google-photos-download-multiple-photos-apk-teardown-3547199/
On Android there is no option to transfer multiple photos from Google Photos back to your camera roll.
You can go to https://photos.google.com/ on the phone, select several photos, tap 3-dots > Download.
They will be saved as a ZIP file, for example in the "Downloads" folder, where you can "extract" them.
You can do the same with an album (or add the photos to a new album): tap 3-dots > Download all.
You can "Share" selected photos with Google Drive and download from there, but that will probably double the storage used by the photos. Therefore not recommended.
Similarly you can share with "File manager +" when you install that app, and download to the folder you want.
You can use Google Takeout: go to https://takeout.google.com/settings/takeout in a browser
You will also get lots of .json files which you can delete. I recommend to try with a small album to start with.
Deselect all products, then select Google photos
Tap "All photo albums", then deselect all
Select an album, for example "Photos from 2022", and tap OK
Scroll down till the end and tap "Next step"
When settings are OK, tap "Create export" and wait till ready
When ready tap "Download" and Unzip
You can download the photos on a computer, connect the Android phone with the computer (USB cable) and copy the photos to the device. See https://support.google.com/android/answer/9064445.
See more options on https://www.wikihow.com/Transfer-Images-from-Computer-to-Mobile-Phone
If you install the Google One app there used to be a one-time option to restore up to 200 most recent photos to the device. This option is no longer available in the Google One free app.
Also note that it is not really safe to delete from your device without making an extra backup. See this article.
It is also possible to download to a computer, connect to the phone by USB cable, and copy the photos/videos.
Advantage is that this gives you a backup, which I think is absolutely needed for safety.
See Google Photos is NOT a "Backup" of Your Photos