String sorting with find function for substring

sir i am unable to understand few things in the code, please tell me. Like find function gives index of first occurrence of a substring in other string , but how it is comparing all characters of substring . otherwise it would check for first similar character then swap for large length first without comparing other characters. Eg :
avvv
ab
link --> https://ide.codingblocks.com/s/574611

It matches the entire subsequence and not just the first character. You can find more information about the find function here: http://www.cplusplus.com/reference/string/string/find/

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