No. A strange thing can be many different things. It could be a rare natural phenomenon, like a ball lightning which looks very strange but is a scientific mystery not a ghost story. It could also be an odd behavior of an animal that we don't understand at first sight, but has a biological explanation.
Fear and the unknown. When a strange thing has no immediate explanation and it induces fear in people, it can easily be turned into a ghost story. For example, if people hear unexplained footsteps in an empty old building, they might start to think it's a ghost because they are scared of the unknown source of the sound.
I don't think it's true. Usually, such stories are made up to engage the audience and spark imagination rather than being based on real events. Maybe they draw inspiration from common human experiences, but not specific real-life incidents.
Well, 'The Strange Thing about the Johnsons' has a real story that's quite hard to stomach. It shows a situation where the power dynamics within a family are completely warped. The father - son relationship is not what one would expect in a normal family. Instead, it involves elements of abuse and a very strange form of control. This makes the viewers think about the dark sides that could potentially exist within family units, even if it's a very extreme and uncomfortable portrayal.