I am a beginner in Photoshop and trying to emulate a template, but now I'm stuck. I don't know how to name the procedure marked by the arrow. So I have a picture which is masked by another layer. Can you tell me how to term it, in order to find a related tutorial or tell me how to do it?
The "square" layer is a Shape Layer. The "picture" layer is a Smart Object. I've created a similarly structured document, to demonstrate this.
I created a black rectangle as my Smart Object and placed a rounded rectangular Shape Layer below it.
Right-click (CTRL + Click on Mac) on the Smart Object (your "picture" layer) and click on Create Clipping Mask.
You will notice that the Smart Object I created (your "picture" layer and my "Layer 1") will now have the Shape Layer beneath it applied, as a Clipping Mask.
If you were to right-click on your "picture" layer and select Release Clipping Mask from the menu, you will see that the Smart Object will lose the applied Clipping Mask and be displayed at its normal aspect ratio.