I think I found the trick for bulk selecting target layers, just by chance...
Click on the target box, then click on the layer, press ALT while selecting.
It is shown here:
THE BEST WAY™️
I've found the best way to do it right now is:
Select all objects in the layers panel.
Click the little menu of the layers panel (top right, hamburger menu)
Press "Collect to New Layer"
Now select the entire layer by clicking the little circle of it.
CTRL+G (or CMD+G on Mac) to group
Now you can move it out of the layer or leave it....
I'm not a Krita expert, however right clicking a layer and then choosing Select Opaque seems to do what you want. It appears to be similar to GIMP's Alpha to Selection, or in Photoshop Ctrl+clicking on a layer to load it as a selection.
Selection by color isn't sharp because at the edge the color often changes gradually. You must bite the bullet and draw a path around your shape. It makes as sharp selection as your photo allows. There's plenty of tutorials available like this: https://www.youtube.com/watch?v=SqUineYZmpA
BTW I have made numerous selections in photos to extract items and ...