I use the Ulysses app to write on my phone primarily. I have about 100 drafts of stories in various states of completion at any given time.
Then I usually do final editing, images, etc. on my laptop (using the desktop version of the Ulysses app). It allows me to write once and then push the stories to publish on Medium and on my own WordPress blog.
As mentioned in the other comment, I do sometimes get into a writing flow state in the Medium mobile app and write a story start to finish on it.