What's typically wrong with cheap mountain bikes? I'm interested in buying a mountain bike. Recently, I was at a Decathlon and checked out their mountain bikes. They only had 3 fullies and I really liked the 700 Eur one. I didn't buy it outright because the dampening locking mechanism seemed to be broken as I was still able to push the handle down even after locking. Maybe you can tell me whether that's normal for any dampening locking mechanism / normal for cheap ones / definitely broken. Here's a short video of me pushing the handle with various amounts of force but generally the same amount of force before and after locking: 
                
             

I'm not sure whether the one I found on their website that one because it looks different (maybe because it's a different color) but this one is the only 700 Eur fully they list.

I didn't only check out bikes at Decathlon but I also looked around bike shops when I was touring around or just cycling in town and came by one. I don't think I ever saw a fully that cheap, and anything that I'd figure comparable would've cost at least triple that.

Now I know very little about bikes. I never bought a new bike and never ever bought a mountain bike. I used the very old mountain bike my father bought some 20 or more years ago and didn't need anymore. But it broke down recently and is in disrepair, so I'm looking for a new one. It did not have fancy things like a dampening locking mechanism, so I have no experience with that.

What potential improvement is there when spending double the money? I don't wanna spend triple to money but double would be okay. I've got a feeling that I'm really missing something here but even after weeks of thinking about it, I still can't put my finger on it.
A fully for 700€ will be horrible. Bad suspension, heavy, few gears, bad wheels, bad tires …

At least get a hardtail where 700€ will buy you a halfway decent front suspension (where the lock-out actually works) and okay-ish weight. With wide, tubeless tires and good riding skills you’ll barely notice the absence of a (bad) rear suspension.