How to feather edges of layer

Using a layer mask works best if you make the mask a little bit smaller than the object.

So Ctrl/Cmd click the object to load it as a selection, and Contract.  The Conextual Task Bar is very handy for this.

Click on the Layer Mask icon and add feather fropm Mask Properties.

If the layers are combined, then create a workpath that follows the edges you want to blur, and right click the path and choose Blur.  You need to set up the blur tool cursor size and strength before dsoing this.

The effect will be a lot more subtle than a feathered mask, but having stroked the path once, you can click on the Stroke Path icon several more times to to increase the feather.