Just make sure you don't need to go at night, since many (most?) are only open during daytime.

according to the link, a third of them is open 24/24

Ok, that's not been my experience, perhaps I just don't know how to find toilets en français

Are they free to use?

The rounded ones in GP's link are free to use, yes.

There are some public restrooms which aren't, mainly around tourist areas, but there's usually a sign and an attendant. The ones in GP's post are fully automated.