Q: How do I add just one photo to an existing album

From Where?

In what app? In either of the you can drag and drop. In Photos you can select the pic and the click on the + button, select Album and you'll have a choice of creating a new album or adding to an existing one.

Be sure you click on the + sign in the toolbar at the very top.  the + at the top of the "moment" where the photo is located automatically chooses all the photos in that movement.  It drove me crazy because all the instructions said to click the plus, which I did but it has to be the plus in the toolbar at the top.

THANK YOU!  I had the same issues.  I get it now - the plus sign at the top of the page vs. the plus sign in the 'moment'.  Really appreciate your posting this.