Copy all visible elements from different layers into one

Open a design that is created with the help of multiple layers.

Each layer has got different patterns in the example shown below

Now select the entire canvas or a part of it then go to Edit >> Select All ( no matter on which layer you are on )

Go to Edit >> Copy Merged

Add a new layer and paste it.

You can also paste on a new file, Note that the new file will have the same dimensions as the copied file.