Apparently this is a flat roof, maybe a terrace roof.
Since the roof is flat, there is no slope to let gravity get rid of the water for you. So it really needs to be done properly, with an EPDM membrane or something of the sort.
If this is not done properly it will leak. From the pics this one is leaking everywhere along the edge.
If the membrane (or whatever else is supposed to make it water tight) is hidden below something, like tile, gravel, earth, etc, then you'll need to get rid of that to access the leaky membrane.
Then, well, it's difficult to find the leaks from above. So given it's leaking pretty much everywhere, you'll probably end up having to redo the whole thing.
I'd recommend finding a contractor who can do that, and asking them how much that will cost. Don't expect it to be cheap, because it's a lot of work.
Do not ignore this problem
You could ask the builder to fix it, but since it'll be expensive, will they really do it?... They'll probably just wait for summer so it dries and sell it as-is instead.
Also there are other tell-tale signs that this building is pretty low quality, so beware. I wouldn't be surprised if all the paint on the exterior walls falls off way sooner than it should ; the little wall in front has wet spots on the bottom which hints that the drainage on the terrace is not working (or there is none), etc.