"s:str" represents that the given paramater is a string and "-> str" represents that the function returns a string

I tell you

can you explain what the first line where you did (s: str) -> str: is doing?
what is the : and -> doing?

Damn, everyday this site makes me feel dumb as hell. Motivating as hell.

hey, curious as to what the return line is exactly doing?

for j in range(i, len(array)):

It is creating a for loop that iterates from i to the length of the array. It is used to compare i to all of the remaining elements in the array.

There are no letters in the example! But when checking the kata, str is used.

what is line 4 doing?

Not a kata issue, comment the sample tests until you find the one your code fails with an infinite loop, read this too: https://docs.codewars.com/training/troubleshooting/#timeout

You are returning the function itself, instead of calling it.