One of the most memorable parts can be the interactions with strangers. When you are on a solo journey, and you meet someone who changes your perspective. For example, I met an old man during my solo travel. He told me his life story, and it inspired me to look at my own life differently. Also, the solitude itself can be memorable. Just spending time alone in a beautiful natural setting, thinking about life, is very special.
The most memorable parts in a solo true story often involve challenges and how you overcome them. Let's say you are on a solo backpacking trip and you run out of food. You have to figure out how to find more. That struggle and the solution you come up with are things you'll always remember. And the unique experiences that are only possible when you are alone, like watching a beautiful sunrise by yourself in a remote place, are also highly memorable.
In a solo true story, the most memorable part could be the moment of self - discovery. For instance, when you overcome your fears alone, like climbing a high mountain by yourself. That sense of achievement stays with you.