I am currently using the Notes app on iPhone to write stories. This started when I wanted to practice my Chinese writing skill. Basically, I outlined my story in my notebook. The story outline was based on a novel plotting video on YouTube. I outlined the novel in English. Then, I picked up my phone and used the Notes app to write the first scene. I sent the first scene to my aunt privately on WeChat; she proofread the work. Then, I shared the first scene in a family group on WeChat. I am writing the story sections scene by scene, and sharing the stories scene by scene, while making changes to the original outline. Here's what I do: memorize all the key plot points, visualize the key plot points, move the main characters along the key plot points, and add details to make the characters seem real to the reader. I have to admit, writing the dialogue, action, and scenery really shows to me what the characters are like as the story progresses. I am also a heritage speaker of Chinese. Even though I was born in China and spent my early years there, I never grew up in China. My parents and I immigrated to the US, so I went to elementary school, middle school, high school, and university in the US. Only recently, I decided to learn Chinese. Because I had knowledge of the spoken language, learning Chinese for me was mostly about associating speech to writing, as well as recognizing that speech is not writing (some spoken words may not have written equivalents, or if there are, native speakers may be unaware). I just like the simplicity of Notes. It is a very simple tool. The primary purpose is probably to take notes or something, but I use it to write a story. Now, I am not really sure if I am writing a novel, novella, or short story. In Chinese, a novel is 长篇小说; a novella is 中篇小说; a short story is 短篇小说. All of them are 小说, differing merely in length. 小说 is too often translated as "novel", I think it is best translated as "fiction". So, I am writing fiction in Notes.
You may want to take a look at things like iA Writer or Ulysses. They add a bit of writing specific functionality over a simple notes app, like the ability to combine smaller texts into larger ones, and tools like word counters and such. (They're both commercial apps and, functionally, quite similar. iA Writer is cheaper and has no subscription, ~$30 one time for the desktop version, phone is cheaper. Ulysses is ~$40 per year.)