That is something I've wondered too. I imagine foundations, elevator shafts and other key parts might still be concrete. Buildings like this are fascinating: https://www.dezeen.com/2019/03/19/mjostarne-worlds-tallest-t...

Burned wood acts as insulation for the unburned core so large timbers can have long fire ratings. The rule of thumb is 1.5" per hour. [0]

I can’t comment on reinforced concrete, but structural steel requires protection to meet 1+ hour fire ratings.

[0] https://www.nishkian.com/fire-rated-wood-construction/