1. The Smart TV is seeing these when the devices are on and have something to share via an app or media server.
2. Samsung and other makers are trying to make it easy (and there are long discussions...) and in so doing create more questions than they answered.
You could on some routers enable CLIENT ISOLATION but while that's my answer I fear it can break some things like printing and sending media to the TV.
I have a Samsung Smart TV. Sometimes in the source section (but not all of the time) it shows a source with an email address that is used on our home computer and my husband's cell phone. Why is this source there some of the time and how did it get there?