I want to select several photos in my Icloud photos folder in order to download them .For this I select one photo, then press the apple command key and want to select other photos. But it seems I can always only select only one imag at the time

Thanks for reaching out, FelixFortunatus.

It sounds like you're trying to select various items in iCloud Photos, but having some trouble with how the items are selected. We can help.

There are a few different ways to select your photos in iCloud Photos:

• You can use Command (or Cmd) ⌘ to click and select individual photos one at a time.
• You can use Shift ⇧ to click a starting photo and an ending photo to select those and all the images in-between.
• You can use Command-A to select all photos.

Give those steps a try, and see if there is any improvement. You can also check out Mac keyboard shortcuts to learn more about time-saving shortcuts on your Mac.

Hope that helps!

If you are using a Mac laptop , simply move three fingers from left to right on a trackpad to select photos in photo library folder from Mac photo application , to select few photos hold shift key and click on first photo and next photos see the article Keyboard shortcuts and gestures in Photos on Mac - Apple Support

You could also click the white background between the photos and drag a selection rectangle across a block of photo you want to select. I cannot test this any longer in the system version you have stated below your question (OS X 10.10), but it is working well for me in macOS 10.14 and macOS 12.