Add photos/videos to  albums

Picasa used to upload folders from a computer to albums with the same name, even in batch, and (optional) sync computer folders with Google Photos.  The actual Photos team however did not find it necessary to implement such option.  Adding photos to albums in Google Photos is now always a manual action.

Add photos/videos from Google Photos

For photos that are already uploaded to Google Photos, you can:

Adding from a folder on a computer to an album

Adding from a mobile device to an album

Adding ALL photos to albums

Some users (like me) want ALL photos in albums.  Former Picasaweb users already had ALL photos in albums when Google Photos was introduced in 2015. They could rather easily continue to add all newly uploaded photos to at least one album. It's more difficult when you start using Google Photos with a big collection. 

Which photos are not yet in an album?

Adding a photo to one or more albums does not remove them from the Photos tab.  How do you know whether a photo is already in an album? 

How to be sure that ALL photos are in albums?