when making adjustments to a layer all the other layers are affected also.

I ran into this same confusion when I first started using adjustment layers, so I get why it feels odd. The key for me was realizing that copying the pumpkins to their own layer does not stop an adjustment layer above them from affecting everything below it. I now make sure the adjustment layer sits directly above the pumpkin layer and clip it to that layer, so it only changes the pumpkins. That also explains why your selection-only version worked, since the selection acted like a mask for the adjustment.

That usually happens with an adjustment layer, since it affects everything below it by default. Try clipping it to just the layer you want by Alt-clicking the line between the two layers, so the adjustment only applies to that one layer.

What kind of adjustments?

If you mean using an adjustment layer, it will affect everything below it in the layer stack unless you clip it to the specific layer you want to adjust. To do that Alt+Click on the border between the adjustment layer and the layer you want to adjust (in the layers panel) and a small downward arrow will appear meaning the adjustment is clipped to the layer directly below it.

If you mean something else, please give further info and a screenshot to explain the problem.

Dave