There is a lot of debate and misinformation on this subject. My understanding is this is primarily an aging or elderly issue. For the majority of people, as we age, we mentally decline which worsens health in all aspects. If you are one of the lucky ones who remain sharp until passing, this doesn't affect you as much. If you are more average, the advice is that a social network of some sort will help you as you age.
There was the famous incident involving Gene Hackman. His care taker was his wife. The wife collapsed/died due to Henta virus and he passed a week later oblivious to what had happened to her. If Gene Hackman and his wife had more proactive support network (the kids or friends FaceTime them everyday), maybe this tragedy could have been avoided.
Is it generally understood as feeling (being in a particular state of mind) as being lonely or just lacking social interactions in general, regardless of how one feels about it? Those two things, to my understanding, aren't the same. For example, I never had friends, and generally, I despise people. I have no "social life," and I wouldn't even be able to clearly define what it is without googling. If I lost my voice, I wouldn't notice that it happened for about a week. However, I don't feel like I am missing out on anything. Does that increase my mortality risk by over 30% or not? I only skimmed the sources, but may read it later.