I want to know why I copied a picture to a layer of another picture in PS, and why the picture seems to have been cut off.
There may be two kinds of incompleteness. One is that the pasted picture exceeds the canvas size, and what you see is incomplete, so as long as CTRL+T is reduced a little,
One is that the pasted picture itself is incomplete, that is, you didn't select all of them when you copied it. You can drag it directly to another picture on the layer without selecting it.