Up until Photoshop 21.1, you could select a shape path with the Path Selection Tool, copy and paste that shape into the selected shape path. I updated to to 21.1 today and the behaviour has changed. I think this must be a bug, because now no matter what you do, pasting the shape creates a new layer, it no longer pastes into the selected shape.
Here's a video demonstrating this issue, the first portion shows the behaviour as it has been up until now, the second portion shows the issue introduced in 21.1.