(This won't make much sense if you haven't seen Flip Flappers, plus it sort of has spoilers.)

To follow on my original entry on the sources of the Pure Illusion worlds, here are some additional notes that are really too big to be added on as an update to the original entry.

• episode 3: As of episode 11, the episode 3 desert world is pretty strongly attributed to Sayuri, per @PeterFobian and @B0bduh. Nick Creamer's tour of the Pure Illusions worlds contains a longer explanation of the evidence (and some additional details).

• episode 5: I'm basically persuaded by Emily Rand's argument in Yayaka's world (and a few stray thoughts on Flip Flappers' Pure Illusion) that episode 5's setting comes from Yayaka. And frankly it's just neat for it to be that way, because (as Emily Rand notes) there are a whole lot of thematic resonances and reflections between Yayaka and the setting. It's the kind of thing that makes me slap my forehead and go 'wow, it so totally makes sense'.

  (I'll also note that despite the horror movie overtones, the setting of episode 5 is not intrinsically dangerous. There are no monsters, no deprivation, no threats. If anything, the school is a refuge from the dangerous outside.)

• episode 9: I'm not persuaded by Emily Rand's argument (from the above-mentioned entry) that the world here is (mostly) the twins. I stand by my views that it primarily draws on Yayaka, although I'm willing to believe that it's intended to reflect and draw on the twins as well. Another person also feels that the episode 9 world is likely the twins. But I still feel that the visual resemblance to Yayaka's locker room scene at the start of the episode is too on-point for Yayaka to not be deeply involved.

In a show as deliberately constructed as Flip Flappers is, I can't help but read something into the last-minute revelation about the source of the episode 3 world. What I personally see it as is a message from the creators to us that we're not overlooking clues to where all the worlds come from; for some of them, we don't necessarily have enough information because the show has simply not shown it to us, just as the show hadn't shown us the necessary information about episode 3's world until the last moment in episode 11.

As a result, I don't think any of the remaining uncertainties can be settled with evidence from within the show. If we find out for (relatively) sure, it will be through future interviews with the creators, BD booklet notes, and other external sources of information.

(Apparently the director wanted to add at least some additional things to the BD releases of Flip Flappers, so it's possible that BD versions of episodes will also reveal more things. But I haven't heard anything about that so far.)