From other threads, it sounds like overdrafts have started to be rolled out to a limited number of people. Not sure it has anything to do with iOS or Android, or when they will be made more widely available.
https://community.monzo.com/t/overdrafts-anyone-actually-got-one-yet/16811/9?u=jzw95
https://community.monzo.com/t/announcing-the-overdrafts-preview/16194/265?u=jzw95