1, copy the first floor and enter the access panel.

2. Find the one with the greatest contrast from the three channels of red, green and blue, and this is the one with the greatest contrast in the blue channel.

Step 3 copy the blue channel

4. The shortcut key ctrl+L calls up the color scale and adjusts the contrast to maximize the contrast.

5. Hold down the ctrl key and click the blue copy with the left mouse button to get the selection.

6.ctrl+I reverses the selection, returns to the RGB channel, and selects the white part below the torso with the quick selection tool.

7. Go back to the Layers panel, and just ctrl+J copy.

The branches cut out in this way are very clear and will not be mixed with blue. Of course, you can also use tools such as filters, but that is more complicated for beginners.

I hope my answer is useful to you. I wanted to send pictures, but I haven't reached the second level yet.