@dbb-the-bruce You can also just left-click drag and drop one design into another rather than right-click, insert into design!
But how do you deactivate it? I got it turned like you said, but can't deactivate, either clicking on it the ball/circle or nothing in the right click menu either. I guess I will get rid of the whole design and start over.When you hover over a component in the browser, the ball or circle shows up at the end of the name/box. You click it to activate that component. You can also right-click the component and the first choice is activate. May be other ways to do this also.
The disappeared components may have been sub components of something broken or in some other way "detached" from the design.
The right-click menu has two options "delete" and "remove" they appear to do the same thing but you can really mess yourself up. Check this post out:
delete vs remove
It's in the autodesk fusion forum. I don't think you need an account. It would be a good idea try to get answers there for some of this stuff.