The following relates to the Google Photos app on Android devices.
Some parts dealing with local storage are different on iOS.
The Google Photos app also acts as a portal to see photos that are stored on the device, just like the Gallery on Android or Camera roll on iOS. You may see a mixture of
photos that are only on the device
photos that are only uploaded to Google Photos
photos that are on the device AND uploaded to Google Photos
This mixture confuses many users, and leads to the loss of photos by deleting photos from the device, thinking that they are backed up because they are shown in the Google Photos app.
You see:
Photos uploaded to Google Photos cloud https://photos.google.com for the active account
Photos in DCIM on Android (usually camera photos and screenshots), and Camera roll on iOS.
This cannot be turned OFF.
Photos in folders (may be called Albums in the Gallery) that are set to be backed up to the actual account.
On Android devices you see all device folders (albums) under "Photos on device".
This seems to be missing on iOS.
Unfortunately Google puts everything stored under DCIM in a folder called Camera. I see two folders called Camera: one shows the screenshots, which are in another folder under DCIM.
The web app https://photos.google.com/ shows ONLY what is backed up to that account.
Note that photos may be moved from the Photos tab to https://photos.google.com/archive
In the Photos tab there is not an easy way to see whether a photo is uploaded or is on the device or both.
When the file name shows a path, the photo is on the device, but may also be uploaded.
Photos that are ONLY in the cloud show just the file name, no path.
When you select a single photo and tap 3-dots:
when you see "Delete from device," the photo is (also) on the device.
when you see "Back up now," the photo is NOT uploaded to the actual account
When a photo is stored on the device it shows "Delete from device" in the info (swipe up or tap 3-dots).
When you tap on Delete from device, you get a warning when the photo is NOT backed up. When you select multiple photos and tap 3-dots > Delete from device, you get a warning when some of the photos are NOT backed up.
Unfortunately there is no way to show, for example, only photos that are backed up to the actual account.
In the web app https://photos.google.com/ you see only photos that are uploaded.
Camera photos (on Android Photos under DCIM, often also screenshots) are ALWAYS shown, which is annoying, for example, when you use the camera for both private and work photos. When signed in to the work account you still see the private photos and also the other way around.
iOS has the equivalent of DCIM and all photos are saved there, referred to as the Camera roll.
Photos in other folders (albums) on Android will not be shown when they are NOT selected to be backed up to the actual account. You can back them up manually, but that makes them more vulnerable.
iOS Albums are software resources and invisible to Google Photos.
Using more than one account may partially solve this issue.
When you still use a DSLR or compact digital camera and upload these photos from a computer, and use the phone much less for taking family photos:
Account 1 is used to upload for example family photos from a computer. These are shown in the Photos tab in the app, together with the Camera photos (and maybe also screenshots) on the device. This account does NOT show the other photos on the device, for example from WhatsApp. There is no automatic backup (account 2 is selected for that purpose), Photos taken with the phone can be backed up manually to account 1 when wanted. They are already safe because they are backed up to account 2 anyway.
Account 2 is used for Backup and sync of Camera photos and other selected folders like WhatsApp. The Photos tab does not show the family photos, because these are on another account.
Photos taken with the phone can still also be backed up manually to account 1 when wanted. They are already safe because they are backed up to account 2 anyway.
When you use the phone camera intensively for both private use and work it is less obvious because you will always see all camera photos in the Photos tab. A solution may be to delete, for example, work photos from the device (select, tap 3-dots > delete from device) to have a clean Photos tab with mainly private photos. The better solution seems to use two phones (there are cheap ones with decent cameras).
It is wise to always have an extra backup of your photos, just in case ...