According to the paper, they used a streak camera. It also helps that they only collected 15ns long segments.
https://en.wikipedia.org/wiki/Streak_camera
Check page 3 of the supplementary information available here:
https://www.nature.com/articles/s41377-023-01095-5#Sec16
In figure S1, you can see a schematic view of the streak camera internals. Apparantely the incoming photons hit a screen and eject electrons from it which are accelerated towards a detector. By quickly changing the voltage perpendicular to the direction of flight of the electrons, they are deflected in different directions depending on when they arrived. That is each detected line ("streak"?) will show up on a different row of pixels on the detector.
Does that make sense?
