I suggest to do courses at https://www.coursera.org/. I have made great experience there and definitely recommend using this platform. Digital Marketing is quite large. It's basically doing Marketing but digitally. I advice you to be more specialized in a certain field of Digital Marketing and then adding more and more skills. It's up to you what you are interested in. It can be purely Online Marketing (SEM, SEO, Website, Social Media, Influencer Marketing etc.), Advertising on digital platforms, Content Marketing (Blogging, Storytelling) or Podcast etc.

It sounds like a wonderful idea. I would highlight a mentoring aspect within the app. It would be great if a young person can also get advice from an experienced professional. Otherwise it becomes an app like any other job portal where you just search, search and search and then just apply to the next best job offer.