This is like driving a car without the need to think consciously about the parts of the car or how to operate them.
Driving a car becomes muscle memory after a few years of doing it, and I can imagine learning languages is the same way. Even when learning English, it takes a few years but now when I speak, I don't have to think about what and how to say something.
There are many different learning theories, but let’s talk about three very influential ones: behaviorism, cognitivism, and constructivism.
I find this interesting because I have recognized many concepts mentioned in our textbook from psychology classes I've taken. I think that this theme may continue to come up, since both psychology and linguistics study how people process amd use language.
Even with constant immersion and simplified motherese, it takes babies many months to say their first word and children take several years to develop their first language completely. Let’s say 4-6 years!
The saying that "it's easy for children to learn languages" seems false for many reasons, and 4-6 years could be how long it would take an adult to learn a language completely. It is clearly not "easy" for either children or adults, and regardless of age it will be just as difficult.
Imagine you ask your friend, “How are you?” and they say, “Fine” but their tone is flat, arms limp, and they are not smiling. What would you do? You would probably ask, “Are you sure?”
The majority of communication is conveyed through body language, which is an important aspect to consider. When speaking any languages, body language says more than our actual words do, and that when learning a language, it isn't just about grammar and vocabulary, but also about expressions, and tone.