Most of the movie takes place in the wedge-shaped building located at the corner of Boulevard Saint-Michel and Avenue de l'Observatorie, in Paris. Note, however, that the building looks different in the early top-down shot of the main character running from one end of the roof to the other. That is because the building in real life does not have an open rooftop. The rooftop in the top-down shot was digitally superimposed onto the real-life building. And the other rooftop scenes were shot on an entirely different building. (Also, in real life, the building is much larger than it would seem from that digital superimposition.)
The scene where Sam marks an X on the window is the 28th day - a possible nod to the movie 28 Days Later (2002) directed by Danny Boyle.
The family's dialogue that Sam is listening to in the headset is in Norwegian. The same nationality the actor is.
The movie poster is very similar to the film. 'As above, so below', also set in Paris
The movie's final scene was shot from the roof of the building at the northwest corner of the intersection of Rue de la Verrerie and Rue des Archives. You can tell by the nearby bell towers of l'église des billettes and Eglise Notre-Dame-des-Blancs-Manteaux.