either a last minute dream before death by digestion, or they got transported to another world ( possibly the afterlife? ) or ( my favorite ) their minds have now become part of the monster ( like everything the monster digests gets transported to like the monster's mental world hivemind type shit )
"If you're hesitating over a decision, just pick the easiest one😌" - A genius. Airi, just be Hina's possession forever, let Hina finish school, get a job, and she'll probably let you live with her, you can even slack off as much as you want 😌.