apply adjustments to multiple photos

You can only lift and stamp the adjustments with ⇧⌘C and paste them with ⇧⌘V while the photo is open in the editor. It is an annoying restriction, but you have to copy and paste the saved adjustments one by one, because only one photo at a time can be open in the editor pane.

But if you collect all photos that need the same adjustment in an album and open the first album in the Editor, you can use the arrow key to step between the photo and remain in the Editor. It will be a bit faster than starting from the thumbnails in the browser.

You may want to send feedback to Apple with a feature request - the more, the merrier!

The feedback form is here: Feedback - Photos - Apple