Board Thread:Questions and Answers/@comment-36761202-20180830022309/@comment-29325824-20180830062350

I don't think that's ever revealed. But the main thing about cleansing is that Hollows are creatures of instinct, driven almost solely by their hunger. Considering Arrancar have regained their sense of reason and don't have that hunger anymore, I don't think they're cleansed at all. Rather, it's likely the same thing that happens when a Shinigami is killed: reincarnation.

As for the consumed Souls, I'm pretty sure that most of them simply cease to be once devoured. Hence why Masaki is never in SS, as her soul was devoured and therefore destroyed by the Grand Fisher.

As for Menos, while I'm not sure about regular Gillians, Adjuchas and VL's probably are such a strong merger of all component Hollows that they are either an entirely new being formed from the fusion, or the dominant personality is the only one that gets reincarnated while the others are permanently gone.