how to extend/shorten multiple clips/layers at once?

Your IMG_9366 clip is on the primary storyline, and the others are connected clips. A connected clip is connected to a clip in the primary, so when you extend another connected clip it does not move to the right, as your gif shows. It remains connected to the same place in the primary. This is normal.

If you want the clips on top to move aside to make room for the extended clips, you could try turning each "lane" into a secondary storyline - select all the clips in the top lane, and press Command-G. Repeat with the ones in the middle. Now you will have connected storylines that behave magnetically like the primary does.

You can't do that with multiple clips in FCP, like you can in other NLEs, cause of the magnetic timeline.