Syntax:
INSERT(str,pos,len,newstr)

Returns the string str, with the substring beginning at position pos
and len characters long replaced by the string newstr. Returns the
original string if pos is not within the length of the string. Replaces
the rest of the string from position pos if len is not within the
length of the rest of the string. Returns NULL if any argument is NULL.

URL: https://dev.mysql.com/doc/refman/8.0/en/string-functions.html

