Yes, the famous SNCF resa system formerly known as Socrates still lives! Renowned for, on occasion, packing the pax in from one end of the car, facing backwards.... No one will check which seat you are sitting in, the on-board staff (if you see them) just look to see you have ticket + resa. But if someone subsequently shows up with the seat reservation, you are expected to move back to your reserved place.
On Duplex trains, the downstairs is pretty claustrophobic as there is no door exit at one end. The upstairs is better.
With Interrail (or for most reservations) there is no way to select a place. Reservations through SNCF's own site and apps will allow seat selection, but (Catch 22 spoiler alert), they won't (as mentioned above) do IR or FIP reservations....
The system's been this way ever since TGV's began, so I guess the French have just got used to it! Something to with 'Egalite, Fraternite', presumably?!!!