Speaking fluent Japanese
I'm learning Japanese so, will I just be fluent in understanding it if I use duolingo? If i only learn to understand, what should I do and what other sources should I use?
This has been said a million times everywhere. You will NEVER be fluent with Duolingo alone. As with every other language you have to put in time and effort, study from books, watch media, talk with people etc. The best Duolingo can get you is beginner level, that's all. Even then if you want a decent Japanese course i suggest you to use lingodeer, EVERY lesson has grammar explanation and goes way more in depth.
Here i my google drive if you want resources: