My family took me up the Eiffel Tower when I was very young but I have no memory of that moment. I don't recall a grand view of the city or waiting in line at the bottom. I don't remember taking an elevator or climbing the stairs. I know that I was on the Eiffel Tower and I consider it an achievement somehow. Truth be told, you're just one YouTube video away from having a better idea than I do of what being on top of Paris feels like.
Sometimes I forget entire portions of less old trips too, until I see a photograph. It isn't so much a trigger as it is mere proof. Eventually I begin to remember what that photograph looks like and over time, that one single image reshapes my memory of the experience and becomes a foundation for nostalgia.
Now imagine opening an old, private camera folder and one inconspicuous picture, not yours, had been transfered inside. If it's nothing too crazy, if it makes sense and fits the context, you might easily own the experience without thinking too much about it.
When you dream about past events, this kind of forgery happens automatically. Then, as you dream again and again about these inaccurate flashbacks, the line between what's real and what's imagined blurs even more. I said dreaming, but conscious thought is, of course, just as prone to distortion.
With the rise of immersive VR technologies and machine learning (deepfakes, chatbots, etc.), it might become easier to merge false memories with the real ones. It could be used for fun or as a tool to de-emphasize traumatic experiences or something.