漢字 means chinese characters, but japanese people use word
I don't understand when and in which context word
かんじ is used and why?
I think your question might reflect some confusion about the Japanese language.
There are multiple words pronounced カンジ in the Japanese language. If you type on your computer, you should see a drop-down with many of them (in fact not all possible words with that pronunciation in Japanese).
For the most part, each kanji way of writing カンジ pronunciation is a different word (a large percentage are names).
The most common word pronounced カンジ is 感じ which means a
feeling, sense, impression. It's the noun form of the verb 感じる.
One use of this word is 「そんな感じ」 which is often used to say something is basically (but perhaps not exactly) correct or as a ぼかし表現 (hedging expression).
A different word is 漢字 which as you know means Chinese characters.
Other words exist as well such as 幹事 which means the person or people responsible for an event or conference.