select subject but keep background in file

I'm sorry your edit didn't turn out the way you expected, but I'm not understanding all of the steps in your workflow, what format you used to save your image, or why you were unable to see that the file had a transparent background when you saved it.  I hate to say it, but this appears to be user error or perhaps a lack of knowledge.  We have all been there.

There are several ways to achieve what you want  which, if I understand you correctly, is to adjust the subject of the photo without affecting the background.

You have not told us what edits you want to make to the subject.  So, I will just give you a random example.

• In the first place, no matter what edits you make, it is always a good practice to copy the "Background" layer and work on the copy.  If something goes wrong with your edits, you can always revert to the original layer.  You copy the background layer by simply pressing Ctrl+J.
• The Background layer is the original image, not just the background content.  When you copy it, the new layer is named Layer 1:

• As I am understanding your post, you made a selection of your subject, and then copied it.  Pressing Ctrl+J will copy only the selected pixels.  The layer will contain only the selection and the background will be transparent.  If you turn off the background layer by clicking on the eyeball icon to its left, you will see just the selection and no background.  In general, if the subject layer is selected in the layers panel, your edits will only be applied to that layer.  The background will be unaffected and if you save the entire file, both layers will be saved.  If you turned off the background layer with the eyeball, and saved the file to a format that does not support layers or transparency, such as a jpeg, the file will be saved with the subject on a white background.  That is most likely what happened to you.  But without more details from you, I can only guess.

In any event, here are a couple of different ways to achieve your goal:

• Open your photo in the Editor.  Press Ctrl+J to copy the Background layer.  Select the Layer 1 copy in the Layers Panel.  Make a selection of the subject. For example, use the Select>Subject menu.  Make your editing adjustment to the subject.  In the following example, I have used the Enhance>Convert to Black and White tool.  The edit will only be applied to the selected pixels and will look something like this:

• Another method is to use a mask rather than a selection to apply an edit to only your desired parts of the image.  Masks also use layers and are more flexible than selections.

So, I think the bottom line is that you should copy the background layer before making the selection.  It is not necessary to do that, but is a best practice IMO.  Making the selection before making the copy of the layer results in only the selection being copied to the new layer.  The remainder of the layer will  be transparent.