Can we learn new words just by hearing them ?

After achieving a decent level of fluency in English, my second and target language, I began to notice that I started to pick up words just by hearing them. For instance, I follow a youtube Channel, and I watch its content from time to time. The youtuber, when about to wrap up the videos, always says "peace out" as a goodbye. I had not heard this expression ever before, and a couple of days ago I was chatting with pal, and at the end of our conversation I said "peace out". I was wondering why it sounded right and from where I had picked that up. Fast foward to some days later, watching the youtube channel that I mentioned previously, I realized that my subconcious mind had learned the expression "peace out" from the youtuber.

It feels strange that after getting some good knowledge of the language, I do not need to grind as much as I used to. Does our brain realize after some point that the new language is important, and then it starts helping us out to absorb the language even when we are not consciously learning?. I cannot even start to imagine how quick we could learn a language if we could learn words just like that simple from the beginning of the learning process. It seems that our yield raises exponencially.

To you, what is the level of fluency where we can really brush up our target language just by exposure?


I tried to write this out without extra resource aside from my own knowledge. Could you tell me what my writing level would be?