For me as someone who has never taken actual German courses, the biggest thing that contributed to my fluency was just listening to podcasts in German non-stop. Didn't matter if I wasn't understanding anything for months and months at the start.
I think the listening played a huge role in familiarizing my brain with wide swathes of the language. It made it so that when I learned other things later on, instead of being actually 'new', it was things I recognized and already had a sort of 'feel' for by association, even if I didn't know what it actually meant.
It was really cool watching as I went through a bit of a 'phase-change' at one point where one week I felt like I wasn't understanding more than few words per sentence and not able to actually follow conversations without looking stuff up, and then the next week it suddenly 'melted' and I was able to bridge the meaning between words and was actually understanding and following entire conversations.
My German still isn't perfect, especially my grammar and I probably should take some courses for that though. But I am at least fluent which is great.