This is how to duplicate layer content in Adobe Illustrator. Sometimes when we're in Illustrator, we want to copy multiple things that we may have on a layer. It's much easier to just grab everything and put it in a new layer so that's what we're going to teach you how to do.
Let's go over and we're going to go to our tools panel. Click the rectangle tool. Just going to draw out some rectangles, maybe change their colors. Now, when we have all this stuff, it's all on layer one. If you move to the right in your windows panel, you'll notice that you have layers and then layer 1 is where all of this artwork is. If you were to expand it, you'll notice that it's all underneath here.
Now let's say we create a new layer by clicking the "new layer" button at the bottom of the window. And let's say we want all of this artwork to also be on the second layer. What we do is we can select all of it by clicking and dragging a marquee box around every item or to the right of the layer 1, we see a circle and it says, "Click to target and drag appearance." If we click that circle, it will then highlight everything that's in that layer. We either use or selection tool and highlight them or we just click the circle and then what we can do is command or Ctrl C (depending on whether you're on a Mac or PC) and you copy all the content and then you click to select a new layer. Click the circle of the new layer and then you can Command F and it will paste all of the content on your new layer. So now if you hide that layer, it almost looks identical because everything is the same there, but you'll notice when you move it, all the content that you wanted to copy is now in the new layer. And that's how you duplicate layer content in Adobe Illustrator.