The shortest answer on this question is: in some places it’s legal, in most places it’s illegal. That said, it’s tolerated most of the time, with the exception of certain regional and national parks.
In general, the laws around wild camping are meant for camping cars, gypsies, long-term wild campers, etc. They are not meant to target a hiker who doesn’t make a mess, stays discreet and only stays for one night.
Because of this, they came up with the word bivouac. This generally means pitching your tent in the evening and leaving in the morning, using a small/light tent. Note that the term bivouac can be open for interpretation (especially in other countries).
Sometimes a bivouac is only allowed with a bivy sack, or sometimes not even that. This is kind off a gray zone. However, as long as you are respectful and don’t stay too long in one place (settle down late in the evening and leave early in the morning), you should be totally fine (even when using a tent).
To summarize, you can wild camp legal if:
- You have permission of the owner of the land.
- In certain areas
- With special permission
Now, you might wonder what these "certain areas" are. Well, most national and regional parks in France have their own rules. If they don't have their own rules, the national rules apply (which means it's illegal).
You can find a full overview of the rules in the national/regional French parks in this article that I wrote: Wild camping in France: complete guide.
Actual legal information on a national level can be found here.