morag88 wrote: … I put some screenshots in albums but they remain in ‘all photos ‘
Right-- ‘all photos' means "All Photos." The Library view shows all the pictures in your Library. Deleting a picture from there removes it from the Library, and so it's gone from everywhere. If a picture disappeared from the Library view, then you'd have a hard time finding it again to put it in additional albums. And if it would mean that putting a picture in two albums would take up twice the space.
One solution for you might be to select your screenshot, and choose Share>Save to Files. Then delete the screenshot from the Library. You can see it in Files, but it won't take up space in the Library.
In Photos, pictures aren't exactly "in" an album. When you "put a picture into an album," its name is added to a list of pictures from the Library that display together when the album is clicked, kind of like a music playlist, but for pictures. So two albums can both have the same picture name in their lists (like two playlists with the same song,) and that picture will show up when you click either album, but there's only one picture file-- it's just on multiple lists. And when you remove a picture from an album, its name remains in the lists of other albums, and the file still remains in your Library. You don't get fewer pictures in your Library because you remove one from an album; you just get fewer names in the album list.
Think about music playlists-- would you really want a song to disappear from your music library just because you had put it in one playlist? You need to be able to find it in the Library when you decide to put the same song in another playlist. When songs were on vinyl, they could only be in one place. And pictures aren't pieces of paper any more.
So albums give a specific view of your pictures. The picture of "Aunt Ethel at the Grand Canyon" can be in the "Aunt Ethel" album with other pictures of Aunt Ethel, and it can also be in the "Grand Canyon" album with pictures of other people at the Grand Canyon. And it can also be in a the "September 2015" album with other things that happened then. Each album is pointing to a single file stored in the Photos Library, so having pictures in multiple albums takes up no more storage space. As you can imagine, this is very powerful in organizing pictures.