I wanted to add to the discussion and talk about my personal experience with language learning, particularly English, which was my first foreign language.
TL;DR Immersion only actually works, but it’s an extremely lengthy and inefficient process.
When I was a kid, I liked browsing Wikipedia articles in Spanish (my native language) and I had very basic knowledge of English thanks to my school (think below A1). And I couldn’t help but notice that the Spanish article was a translation of the English one, and that the English vesion had more content! In my 8-year-old mind, I concluded that, therefore, everything in English is factual, more up to date and better than anything in Spanish, so I stopped trusting information given to me in Spanish until I could fact check it in English.
Next year, I switched schools to one that didn’t teach English, but my little fact checking habit persisted, and eventually led to me only watching Youtube videos in English as a teenager (which maybe you can surmise was a considerable portion of my day back then). I only watched videos in English, anime with English subs, video games in English, etc etc, the only Spanish in my life was my family and friends. I did almost no output, besides the occasional argument in forums or Youtube comments. That habit persists up until now, to the point that sometimes my English tends to be better than my Spanish in everything but pronunciation (which is not that bad, and I’m currently working on it).
But that’s just anecdotal, the hard proof is that I passed a C2 test (iTep, I thought it was way too easy, so maybe take it with a grain of salt) because I needed proof of second language skills to graduate from university.
Now, this whole process must have lasted about 8 years until I became truly fluent, so it’s not a particularly efficient way of learning, however, it is effective.
Now for the other side, I’m currently learning German using a mix of LingQ, Anki and the DW course, while generally following the Refold philosophy and constantly mining sentences, and I can say that progress must be at least 10 times faster, if not 100 times. By mining sentences your vocabulary goes up really, really fast and it creates a feedback loop because it’s easy to learn new words. I’ve recently switched to monolingual Anki cards, and it feels really nice to learn new words through their own German definition. And the last thing is the DW course, which constantly asks me reading comprehension questions (also monolingually) and has been a great mix of immersion and studying, because they give you grammar explanations in German. I’m currently solidly at a B1 level and I’ve been studying “seriously” (more than 1 hour daily, currently about 5 hours daily) for about one and a half years. Overall I’ve been studying German on and off for 5 years at this point. Mostly off, because if I had been studying this hard the whole 5 years I’m sure I’d be at least C1.
Also worth noting that I’m doing 0 output besides the DW course exercises.