"Genesis is a Forerunner Builder facility constructed to act as a connection between the digital world of the Forerunner Domain and our physical realm."

And (not very relevant, but interesting I guess),

"The Forerunner grew Genesis-it's a totally artificial planet-so the landscape reflects the aesthetic of their technology in many ways."

: That is the question, isn't it? I think we won't know until Halo 6. I agree
: though that it's not just different, it's out of character, and there's
: not the direct explanation this time that she's just not in her right
: mind, she says her rampancy was cured and she acts stable. It's the
: weirdest and worst part of it to me.

I still think folks might be overthinking this. The fact that Cortana is acting so out of character serves as a huge indicator that it's not Cortana. It's a highly rampant fragment of her that's been even more corrupted by Forerunner AI. Or something like that, or something totally different than that, but it's not the same Cortana we knew.