Yeah this one is honestly still a huge mystery for me too. I remember after Shibuya, everyone assumed she was dead because even Shoko (the healer) said her chances were slim. And then… silence for dozens of chapters.
Then suddenly in Chapter 267, Nobara just shows up alive and nobody really reacts to it. No flashback, no detailed explanation, just “oh hey, she’s back.” That kind of cheapened the emotional impact from Shibuya, not gonna lie.
My guess is Gege just didn’t want to slow down the final arcs with too many explanations. Maybe he thought fans would just be happy to see her again. But still, it would’ve been nice to at least get a short scene showing how she recovered, who saved her, and how she healed from an exploded face.
Some people think it was Shoko’s reverse cursed technique that saved her just in time. Others believe Nobara was in a suspended state between life and death. But again, all speculation.
I really hope the final chapters or a databook clears this up. She deserved more than just a silent comeback