Things like Hacktoberfest are a great way to push yourself to finally jump into OS contributions or to build a habit of submitting PRs. As job preparation and resume strengthening though, one-off PRs are’t a big help. What can be very useful is really getting involved in a project and being a regular contributor. This is much more like what you’ll do when you begin working as a developer. You’ll come into an existing, unique, confusing code base that includes technology that you’re unfamiliar with. You’ll have to work with strangers who have their own ways of doing things. You’ll learn by doing and develop a sense of ownership over the project that you are helping build and maintain.
You’ll be able to demonstrate to potential employers not only that you can accomplish the above, but also a pattern of regularly contributing, adressing bugs, responding to feedback, and providing your own.