Hi Ebuka! I am glad my article could help you.
That’s a good question, I also struggled with it for a while. There are 2 types of publications: owned by Medium or not owned by Medium (owned by private people, organizations, etc). To submit to the Medium owned publications, you can send them an email. You find the email addresses on this page: https://help.medium.com/hc/en-us/articles/360025123674-Submit-stories-to-Medium-s-Editorial-Group
For the other publications, it depends. Some do not accept writers or submissions. But many do. You need to go to the Medium page of that publication and, in most cases, they have some info on how to submit your stories. Many times it’s through a form, other times it’s via email, other times you comment on a post requesting to be added as a writer. Once they add you as a writer, you can then submit specific articles.
I hope this helped! If not, drop here any further questions!