One of my favorite baked-in features of Medium is how its search function works. You can filter by publication when searching any term and it gives (actually) relevant results. From there, many publications have a “Submit” tab or similar at the top, giving directions on how/what to submit.

Then we have the massively helpful resource that is Smedian. You can discover Medium publications there, request to contribute to them, and much more. I would highly recommend checking that out.