Such was the dedication to truth and authenticity in this film, it's easy to assume that every location that's used in the movie was simply filmed in its real-life counterpart. However, that's not always possible. A lot has changed since World War II; especially places that were heavily damaged by bombing and gunfights. That's especially true in France, where heavy modernization and repair work was carried out after liberation. Because of that, it wasn't possible to film the French scenes in France; France just doesn't look like that anymore!
Instead, a specially-constructed shell town was built just outside of London, England, and filming took place there instead. That means every single one of the ruined buildings you see in those scenes has actually been uniquely created to look that way. It's convincing - and we imagine it was also very expensive!