Essentially Sana has become a creature more like what Iza was when she first met him, whereas Iza becomes more human as Sana loses her humanity. In the end I believe it is showing Sana’s total transformation into a “creature” as she is now haunting the people she comes across similar to Iza. It mentions repeatedly throughout the main series, and this one slightly, that anyone who forms a connection with the fox will forever be ‘haunted’ by him and will be attracted to him. Ultimately everyone except Sana(and maybe Arin) are destined to die due to the fox. Idk if you have read the original series, but if you haven’t it would be heavily confusing as you’re missing a whole lot of context. It’s a pretty confusing and surreal story.
Someone just explain the ending to me...