What is R?

The answer is string[] | number[]. Here T is a naked type parameter: it stands alone in the check (T extends ...), not wrapped in something like [T]. A conditional type on a naked type parameter is distributive. This means it runs once for each union member and then joins the results into a union.

So ToArray<string | number> becomes ToArray<string> | ToArray<number>, which is string[] | number[].

(string | number)[] is the tempting wrong answer. To get it, wrap both sides in a tuple so the type is no longer naked: [T] extends [any] ? T[] : never.