I think it took me a while to understand (after being told by native speakers) that Korean and Japanese are a bit of a linguistic evolutionary puzzle, especially if you thought that because their characters look like Chinese, their spoken languages are related to Chinese. They are not. The languages must have come first / evolved independent of China, and then a wave of foreign commerce / scholarly / government influ…
A close analog to the place Chinese has with Koreans and Japanese speakers is the place ancient Latin and Greek have today with English speakers. Lots of loan words, many of which aren't even recognized as such by native speakers, and uses in prestige modes like philosophy, science, etc. One difference is that Rome fell and the Normans brought a highly changed Romance language with them into England. While Korea and…
Chinese language was introduced mostly between 4th and 7th centuries [1]. Japanese had no writing system at that time, so they adopted Chinese ideograms (hanzi) and vocabulary, but changed the pronunciation to fit the much (much!) simpler Japanese phonetics. They also started to use the ideograms for words that already existed in Japanese, with their Japanese reading. That's why Japanese ideograms (kanji) have two kinds of readings: Chinese readings (onyomi) and Japanese readings (kunyomi). To make it even more complicated many kanji have multiple readings, and you have to learn which to use depending on the context or the word it's part of.
Later some ideograms started to be used purely for their phonetic, ignoring the semantics, to better match Japanese grammar. With time these ideograms were simplified and they became two different syllabaries: hiragana [2] and katakana [3]. These are not ideograms, but letters that represent a full syllable. Hiragana is used for grammar particles, to spell out kanji and for some words. Katakana is used for borrowed foreign words, or sometimes as a sort of "italics", to emphasize words.
[1] https://en.wikipedia.org/wiki/Chinese_influence_on_japanese_... [2] https://en.wikipedia.org/wiki/Hiragana#Table_of_hiragana [3] https://en.wikipedia.org/wiki/Katakana#Table_of_katakana