It's not O(n^2) ya doofus. It's O(n * k), where n is number of splits, and k is the select (or vice versa). It's still very time complex, but it doesn't grow by the number of splits squared.

It's just named "sorted", not that it's sorted

Please use spoiler flag next time, your post was visible in the homepage.

It absolutely is an increasing sequence. The kata defined 0 as coming after 9 for increasing sequences. The 1 in the end can be safely ignored because a number was defined to have at least three characters

nah you'r right @VecZz that would throw a DividedByZeroException.

The kata explicitely says the idea is not to sort it... Why is this considered best solution?

As I understand it, it's just how BitConverter works. It seems that whatever architecture CodeWars uses to run our code happens to be little-endian, and thus GetBytes() acts accordingly. See the documentation:

The order of bytes in the array returned by the GetBytes method depends on whether the computer architecture is little-endian or big-endian.

Do note that the Kata does implicitly describe the first octet being the left-most one (and, correspondingly, the most significant one).

nice solution, I just dislike how you've written the IsPrime function. The if that everything is wrapped in could easily be replaced with another guard clause. You did use guards so you seem to know that guards are prefered when using ifs.
how about if input is 2 return true, if input is 1 return false? so that catches the only edge cases I have in mind right now with a nice guard clause and then you can keep your approach without nesting everything into an if check.

I wonder why choosing why choosing a List of tuples over a regular Dictionary?