The following excerpt is from the top answer on 知乎:
The 便当 here means a bento-box. However, since it is clear that "bento" in English is a loanword from Japanese, I feel that it is a loanword from Japanese in Mandarin as well, not a word that had been incorporated into Japanese.
However, the Japanese word is 弁当, not 便当. So my question is:
Is my assumption correct that it is a loanword from Japanese?
If that is true, then why is it called 便当 in Chinese? And when did it start to be called as such?