> I would argue that immersing yourself in a native environment is hardly a simple thing. It's expensive - often in money, but always as an impact to your life. If you have a job, or spouse, or kids or pets, how does this work. I've actually done this and each one of these things is a huge problem. If you're a young, single person who can work remotely or doesn't need a job, then maybe, but that's not most people. If you do need a job, getting a visa to work in a new country is also non-trivial and often impossible.
Too many ifs, sorry but those sound like the same excuses people say when they explain why they can't travel (more). You have to figure out what your priorities are, and if you really want something you'll find a way.
>It's also not just on you to "force yourself to speak a language". You need someone to listen to you.
Easy. Just go buy some food. You don't need much language skills for that task and it's a start. Or if you are single, go on a date with someone who doesn't speak your language (this was personally the most efficient way for me personally). You overcomplicate things. Your rate of learning a language is proportional of how uncomfortable you want to be. I've seen people studying in language schools for years until they get everything perfect and they can't even maintain a basic conversation. Overall this idea applies to all learning: "to become a master you should be willing to be a fool", i.e. make mistakes.
> It's also very hard to pick up vocabulary implicitly from conversations unless you have some base. My German is good enough now that if I don't understand a word I can ask them to rephrase it using other words and I can often get it. Children can do so just from environmental and contextual input often, but it takes them _years_. So you still have to spend a lot of time studying and building vocab on your own in order to try out.
Most of the information is communicated via non-verbal channels: body language and intonation for example. Using your intuition is crucial here. The rest is basically pattern matching. A single word you can understand from the context. Once you understand > 50% words in a sentence, you can derive the rest from the context and then it becomes a self-reinforcing learning system.
Well, children also don't have the same intellect as an adult and cannot apply more sophisticated learned cognitive techniques.