Same but select more albums in the middle
- Select one album with a right click (or long press) as above
- Click on several others (left click or regular press), you can scroll back and forth as you select albums; you can do that in search results or artist album listings or wherever
- Do the action as above, delete or play or edit metadata or add t9 playlist or favorite or…
In Windows (and I assume Mac) you do something similar but you have to hold down a key while you multi-select, requires a certain finger agility
We’re getting there. Checked on the macbook and iPad and for the Macbook indeed it’s holding the CMD button.
After the selection in de edit-box you can delete.
Now…still left is the adding of this multiple selection to an existing bookmark
That’s not possible. A bookmark saves a specific view on your library (albums without genres for example). You can not add albums/tracks/… to a bookmark in other ways than make them comply or not to the filter/focus criteria(s) saved in the bookmark. Use tags or playlists instead if you need the ability to add albums/tracks/… to something.
FAQ: How do I create a smart playlist? How do I save a Focus?
As @BlackJack says, a tag contains albums (or other things).
A bookmark is just a view… for example, of a bookmark 
What do you mean? Changing the behavior of bookmarks in a way so that it’s possible to add albums/tracks/… would make them something like tags or playlists but no longer bookmarks (saved views).
For my example above, edit selected albums and add a genre or delete all genres to add or delete them from the bookmark.
Eg. In my album view I want to select a group (but not all) female jazz singers and after that I want put them in a(n) (existing) bookmark called Female Jazz Singers…or in a new bookmark
What filter/focus criteria is the bookmark “Female Jazz Singers” based on? If it’s a tag, then add the selected group to that tag. If it’s a genre, then add that genre to the selected group (use Edit… from the 3-dots menu or the Edit button top right to do so).