Japanese Kanji (Chinese ideograms)
To fully understand how kanji appeared in the Japanese language, one must try to place oneself in 7th century Japan.
At that time, the Japanese already knew how to speak the language, but did not have a writing system. On the other hand, China is a very influential country with a technical advance superior to other countries. And it is little by little that the Japanese soak up Chinese culture and end up adopting their writing system, namely Chinese ideograms or kanji.
But, the ideograms are not really adapted to the Japanese language, which is why the Japanese started by using them not for their meaning but for their pronunciation. That is to say by trying to use Chinese ideograms that are pronounced in an identical or approximate way in relation to their language.
Towards the end of the 8th century, the Japanese began to move away from the Chinese and their culture. This withdrawal will allow them to assimilate foreign contributions to adapt them to their taste and culture. Indeed, most of the Chinese ideograms used included a large number of lines. It was therefore tedious to write by this method. This is why, over time, two new writing systems will be born based on the phonetic writing of Chinese ideograms called kana.
Resulting from poetic improvisations of the aristocrats of the time, they were born from the increasingly stylized writing over time of Chinese ideograms and end up losing their link with their original character. Formalized in 905, they are now used to write words of Japanese origin (those that existed before the introduction of Chinese characters), but also words introduced over the centuries (composed of Chinese characters) which represent more than 60% of Japanese vocabulary. They are also used to transcribe the declensions of verbs and adjectives.
• The katakana
Coming from a different evolution, they were created within the great monasteries in order to simplify the writing of Chinese texts. Each katakana retains only a few bars and dots of its original ideogram, but nevertheless retains its phonetic value. Nowadays, they are used to transcribe words of foreign origins (Westerners for the most part) and onomatopoeias. In much rarer cases for stylistic reasons, they are also used to write words of Japanese and Chinese origin.
• The romanji
(literal translation: roman meaning roman and ji character: Roman characters) are used in most cases so that foreign tourists can navigate (but beware this system is mainly implemented in megalopolises). Romanji are also widely used for company names, anagrams, or other abbreviations. A new current of Romanji is in fashion, this one aims to make "trendy" (especially in restaurants, bars or other cafes ...) So thanks to this new wave, more and more people like to write a few words in Romanji, especially in French, for example to personalize your sign, or even decorate your interior design.
Japanese kanji dictionary
Jim Breen's Dictionary