I think you need a combination of both. Just working on random projects isn't feasible all the time. I've worked on projects where I simply did not know how to

I think you need a combination of both. Just working on random projects isn't feasible all the time. I've worked on projects where I simply did not know how to complete said project and googling didn't help much. There is real value from taking a voluntary formal course that is comprehensive. It will give you the basic foundation to master the subject. You can use projects to refine your skill.