One thing I can't figure out is how to add to or subtract from an automatic subject or object selection. I can click add or subtract and use the ebrush tool, but it seems t make a different layer. I want to invert my TOTAL selection so I can work only on the background, but it doesn't seem to combine the selections.
The different layers are the whole idea behind the new tools. You have a main mask, that is built up from different submasks (Adobe calls them 'components'). By having different these components or layers, also brush refinements become non-destructive. You can undo a brush refinement at any time, without having damaged the layer below. With the old tools, if you brushed over a graduated filter for example, and later changed your mind, you could not get the brused-over graduated part back. If you erased the brush, you would end up with no mask, bot the original graduated mask.
If you want to invert the total mask, then do the following. Invert the subject mask, then right-click on the brush mask and change it from add to subtract, or vice versa. Do not invert it too, that does not work.