I can't believe that many Frenchmen voted for the Communist scum, but I think it's a alliance of the old Communist trade unions and the New Left open-borders, college educated identity crowd.
If you look at the electoral map, it's very similar to here: urban cores and industrial areas vote hard-left with some Macroniste public employees, Le Pen takes most of everything else with some smaller right parties:
France almost went Communist after WWII.
They were purging their opponents in Paris after the Liberation, by labelling all their enemies as Nazi Collaborators, and having secret trials, before DeGaulle put a stop to it.