Codewars is also here to showcase actual best practices and this is one of the rare cases where the votes match.

ehehe I like it

It certainly looks elegant and functional, but I dug deeper into how Sequence works and how effective they are for small sets or sequences. As a result, using the measureTimeMillis function, I found that executing generateSequence with subsequent processing takes from 5 to 13 seconds. While if you use "do{}while(...)" or just "while" it takes 0 seconds. From this we can conclude that although Sequence is very concise and saves us from mutability and temporary variables, we still need to be careful when using them for such tasks.

I think this is clever but definitely not best practice.

could i ask how one would go about learning about functions that can do this stuff because this is the first time ive encountered it

How to make the oper function accepts only methods of Opstrings?

worst solution. this is codewars, you should write your own logic and not use existing solution

.take():
Returns a string containing the first n characters from this string, or the entire string if this string is shorter.
example:
"Hello World".take(5) would be "Hello"

what is .take() function?

sumBy is deprecated use sumOf