A function to view all photos that are not in any album (sometimes called Orphan photos) is one of the long requested features. Adding photos to albums is a way to structure a photo collection, and find photos more easily.
The photos team has never implemented that function. Note that in Picasaweb, the original Google Photos app, ALL photos were obligatorily added to albums. Since Picasaweb albums were converted to Google Photos albums, Picasaweb users had an excellent starting point, but needed to carefully keep adding all important photos to albums.
The info of a photo shows the albums to which a photo has been added. Till end 2024 that was only the case in the web app, but has now been added on Android and (I guess) iOS.
That allows to check whether a given photo is in any album, but not useful for a systematic check.
For users not having too many photos (in any case < 20.000) there may be a work-around:
Add all photos from all existing albums to a new album, for example called "Photos in albums".
Note that the same photo will not be added twice.
Open the new album, and sort by "Recently added" (3-dots > Edit album > Sort)
Note which photo is on top (date/name)
Click on "Add photos"
That brings you to the Photos tab where all photos that are in the new album are "selected".
Be careful: deselecting a photo removes it from the album.
Not-selected photos are not in this album and thus not in any album.
Select the not yet selected photos: that adds them to the album.
Since the photos in the album are sorted by "Recently added (first)", the photos higher on top than the one under 3. are the ones that were not in any album. You can now add them to a new album for later treatment.
I tested this on a test account with only a few 100 photos, and it seemed to work, but of course will be more laborious or impossible with too many photos. Only for relatively small collections.
Github Google-Photos-Toolkit
https://www.reddit.com/r/googlephotos/comments/1dn4sq8/google_photos_delete_all_photos_not_in_an_album/
Select all albums in the 'exclude albums' filter, then use 'move to trash' action.
https://github.com/xob0t/Google-Photos-Toolkit
Suggest more options
If you know other (better) solutions, please suggest them in a new thread on: https://support.google.com/photos/thread/new
Users of digital cameras can select photos they want to keep on a computer, and manually upload them directly to an album on https://photos.google.com/albums. See Add photos/videos to an album
Phone users have a problem when all photos are backed up automatically ("Back-up" is turned ON). They can add the photos they want to keep to albums, but have problems to find and delete the ones they do not want to keep.
Of course you can delete (move to bin) the really bad ones immediately, but making a nice album is more than that: you want to tell a story. After this first step there will still be photos you do not want to keep: near duplicates, less important photos ...
Select one or more photos in the album
Tap 3-dots > Move to bin
That avoids a second action and mistakes.
Users like me, still using a good digital camera for the important photos, upload them manually from a computer to an album on a main account A. To prevent pollution of account A, I backup photos from the phone to another account B. That is a mixture of whatever I receive, for example via WhatsApp, and more and more photos I take with the phone. In 2024 me and my wife even happened to use only the phone during 2 vacations. Times are changing!
I kept the habit to select photos, now in account B instead on the computer, and share the good ones (by link) in order to add them to an album in account A. The same happens with my wife's photos, or photos from other sources.
Select a second account B for automatic backup to keep account A clean.
Share the photos you want to keep with account A.
I use to share an album. Photos shared in a conversation cannot be added directly to an album !!
Open the album while signed in to account A
Select the photos, tap +, and add them to an album. If possible, I do this on a computer.
In the web app, you get the choice between "New album" or an existing one.
On mobile, tapping on "Album" immediately creates a new one !