Pumpkin
“But once Rausch was definitively gone for the day, the office transformed itself as instantaneously as a pumpkin into a carriage. Music was turned on (they rotated among the fifteen of them who got to choose), and takeout menus materialized, and on everyone’s computers, work for Ratstar Architects was sucked back into digital folders, put to sleep, unloved and forgotten, for the night. They allowed themselves an hour of waste.

How to understand "pumpkin into a carriage" ?
I understand it, say that, after confirmed that the boss had leaved, the staff immediate highing.

Question

Pumpkin
“But once Rausch was definitively gone for the day, the office transformed itself as instantaneously as a pumpkin into a carriage. Music was turned on (they rotated among the fifteen of them who got to choose), and takeout menus materialized, and on everyone’s computers, work for Ratstar Architects was sucked back into digital folders, put to sleep, unloved and forgotten, for the night. They allowed themselves an hour of waste.

How to understand "pumpkin into a carriage" ?
I understand it, say that, after confirmed that the boss had leaved, the staff immediate  highing.

Answer

It's a reference to the fairy tale of Cinderalla, in which the fairy godmother transforms a pumpkin into a glittering carriage so that Cinderella can travel in style to the ball.

Answer

This is a reference to a classic fairy tale called Cinderella. In the story, Cinderella's fairy godmother appears to help Cinderella go to a ball by changing a pumpkin into a carriage, mice into horses to draw the carriage and her rags into a beautiful ball gown. She did this all with just one wave of her magic wand. So the author is saying that just as quick as the wave of the wand, the office became transformed.

Answer

Which book are you reading?  You seem to be translating or annotating the entire book.