You mean a mountainbike with that right? I think your choices are very limited at that price range. Maybe walk in the nearest Decathlon and see what you can get.
A new mountainbike that is at least slightly useful for mountainbiking, will cost more than 300€. I would say it starts around 1000€.
If you only plan to ride in the city or unpaved dirt roads then any normal bike will work. Anything more wild will quickly destroy a 300€ MTB. (spokes and rims snap; brakes are too weak; heavy weight; cranks, seat post, handlebars or even frame can break)
Basically all brands sells bikes in all price and quality ranges.
There are 3 or 4 relevant producers of components and frames mostly come from a handful of factories too.
Cheap bikes tend to have lots of flashy features that appear impressive but none of it is good quality. For example cheap suspensions are heavy, develop play/wiggle and are non-serviceable. Adjusting cheap no-name brakes or shifting is frustrating.
With that budget I suggest to look at used bikes. You can get a nice mountainbike from 1980s to 2000s plus replacement parts for that price.
Certain models and brands are more sought after by collectors because they are considered "classics" so you would basically pay for the name.
Giant bikes are imo good quality but without the cult-factor that drives up the price. If you want to avoid fixing a bike yourself then some shops also sell fixed-up old bikes. In Germany there exists "bicycle flea markets" where people sell their bicycles and the organizers provide some experts that buyers can talk to. Maybe that exists in France too?
For countryside gravel/dirt roads this will suffice.
The thing is that mountain bike competitions are suicidal jumps down the rocks, and the one that normal people need are just bikes with thick tires, that don't slip on occasional mud. And I strongly suggest not snubbing wheel fenders, like on urban bikes (I did snub, and had to wash the bike every day ), because the only case you don't fenders is downhill race. Fenders like on cross motorbikes, are poor protection, and a waste of money for posers (I was one).