Question

printf("enter the string");
    scanf("%[^\t\n]s",str); 

//how the abv code works to accept multi word string without using gets and only using scanf

Answer 1

[ Matches a nonempty sequence of
characters from the specified set
of accepted characters; the next pointer must be a pointer to
char, and there must be enough room for all the characters in
the string, plus a terminating null byte. The usual skip of
leading white space is suppressed. The string is to be made up
of characters in (or not in) a particular set; the set is
defined by the characters between the open bracket [ character
and a close bracket ] character. The set excludes those
characters if the first character after the open bracket is a
circumflex (^).

…

The string ends with the appearance of a character not in the (or, with a
circumflex, in) set or when the field width runs out.

(Linux scanf(3) manpage.)

So scanf reads up to the first character that is not a tab (\t) or newline (\n). It does read spaces.