Who redacted the Talmud? The Gemara famously says "Ravina v-rav Ashi sof horaa", "Ravina and Rav Ashi were the final decisors", (Baba Metzia 86a). That's as close to a statement about this question from within the Talmud itself. Ravina died later than R. Ashi, in the equivalent of about the year 500 CE, according to R. Sherira Ga'on.
Clearly, it is unlikely that Ravina himself wrote "Ravina v-rav Ashi sof horaa". So who did? According to R. Sherira, following the death of Ravina rabbis who could no longer be called amoraim but whom R. Sherira calls saboraim continued the work intitiated by R. Ashi and continue by his student Ravina, the final compilation and redaction of the Talmud. The Gemara quotes by name the opinion of saboraim (a term, which incidentally, is found in the Talmud Yerushalmi) . R. Achai, for example, is thought to have been a sabora. Although the saboraim considered there to be a marked distinction between themselves and the amoraim, evidently that didn't preclude them from adding some of their own, by name, into the mix.
In short, the saboraim are a short shadowy group with no precisely defined role. Their existence is surely acknowledged by all, but the popular and traditional view of the Gemara scarcely makes room for them. How much of the Gemara was written by them? There are minimalist and maximalist views; a lot? A little? Does it matter?
What I think is interesting in itself is the fact that history doesn't really record the appearance of the Talmud (although it is unsurprising for its time). It was just there by the time later geonic authorities were commenting on it and about it. Who wrote it? When? There are educated guesses but the silence is deafening.