You can take a tour to understand what StackOverflow is:
Stack Overflow is a question and answer site for professional and enthusiast programmers. It's built and run by you as part of the Stack Exchange network of Q&A sites.
And you can know more about hashnode in about page:
This is a safe place where you can discuss your ongoing projects, write blog posts about up-and-coming trends in software development, share what you’ve learned with the rest of community, get career advice from fellow developers, and provide feedback to others in a non-toxic, friendly environment.
Everything else, I would say StackOverflow is like a hen and hashnode is like an egg for professional and enthusiast programmers. You might disagree with this. But I will insist on this because hashnode is just an egg in its growing age.