If you were given the task "mimic the sound of Japanese as best you can", at first, you would just learn the basic phonology and just try and mimic the general sounds of the language. You would get good at that, and eventually you would be perfectly mimicking the pronunciation of Japanese phonemes. At some point, it would become worth your time to actually learn how different sounds are put together, e.g. Japanese can't have an S sound followed by a T sound. After that, you may start to learn how the different syllables interact. And at some point, you would learn how words are put together to form sentences.

At a certain point, there really is no difference between "being really good at mimicking the sound of Japanese" and actually knowing it, because in order to mimic it to a high level you will have to actually know it. "Mimic the sound of Japanese" is the equivalent task here to "predict the next token in this text".