As a native Russian speaker who lived abroad for most of my childhood (Malta), I got to skip formal Russian grammar lessons in a real Russian school. I went straight to university, where Russian grammar testing was no longer a thing.
I got a 37 out of 100 in my Russian ЕГЕ exam and 33 (hard fail) in my Literature one back then, but 99 on my English one (that's how I got into university). Anyway, I can, with much confidence, say that I have absolutely no idea of any of 99% of the rules in the Russian language and still make really stupid mistakes (I'm turning 33 in a month and lived in Russia for almost 20 years now) when writing and speaking and still have a tiny weirdness around Russian accent as well :) and I forget words quite often.
You question got me thinking a bit and I realized that I know absolutely nothing about Russian grammar, I don't know what parts of speech are called, how many cases there are, types of verbs, etc. Yet, the language itself flows quite well, as I've given more than 30 public speeches Russian throughout my life. It's quite funny to realize how I don't actually formally know the language.
I guess the same would be true for maybe 80% of the population in most countries? They don't really know the formal grammars of their languages, yet they're able to speak them natively and even pass exams. You still see native English speakers use "their" and "there" interchangeably, I guess it's like our "недевать" and "одевать" type of thing that's rooted in culture somehow.
Either way, I think that speaking is actually the most effective way to learn a language for applied use. Spending a couple of years in a foreign country, especially as a child, will have you pick up the language at a very intuitive level. It's harder for adults.
Continued in comment above... (this 2000 character limit rule is silly)