Self-study will always rely on some material made by others (whether it was specifically made for language education or just content in your target language), and depending on your personality you may be disciplined enough to go through the material on your own, but I think it's very helpful to have a knowledgable guide who can help you understand how to correctly use what you learn. Unless you're incredibly driven and systematic in your self-study approach, and willing to immerse yourself in Japanese language content (like youtube's "Matt vs Japan") you likely won't bear much fruit by yourself.
I think you can learn a lot through self-study, depends on the person and how hard they apply themselves. There isn't any guaranteed or only a particular method, as we all learn differently, so what works best for a person can vary greatly.
Maybe you want to go check out Japanese Language Decoded, which is software to assist those doing self-study. In addition, it can be fun and worthwhile to get into watching Japanese movies, TV shows, and reading news articles.