If you use "exists" in the Buddhist context, you might be understood as "has own, unconditioned essence" which is the traditional use -- here, the object in question cannot suddenly become or unbecome according to circumstances, because it always "is". Or as "is real / refers to an actual experience" which is the more popular understanding. Not being explicit about how this word is being can be confusing.
Having said that, to the questions:
Does samsara exist? Or does samsara not exists and it just looks to us that it exists, but in reality it is all the same "thing"?
I'm understanding samsara here as a word to describe the cycle of birth and death.
Wether it "exists" -- in the traditional sense -- or "not exists" is a philosophical topic whose answer might change according to the philosophy of each school (though I think no school who would declare existence of phenomena or concepts have survived).
Does this mean, that when you attain Nibanna, samsara does not exist but it exists for other beings in samsara?
from the following:
The Buddha attained nibanna, but here we are, subject to samsara.
we can conclude from our experience that samsara is "real" (insofar as the cycle of birth and death is actual -- or if one understands samsara to be simply this circumstance of being subject to suffering).
In the traditional sense of "exists", samsara either "exists" or "not exists" -- it doesn't make sense to change according to circumstances (eg. when an event happens, like attaining nibanna).
The "cessation of samsara", insofar as we are talking about the Buddha attaining nibanna, is not the cessation of our reality, nor the cessation of the samsara we are subjected to.
Does this mean, that when you attain Nibanna, samsara does not exist, and neither it exists for the other beings in samsara, because Nibanna is unconditioned, so there is no samsara, and consequently there are no beings?
Beings "don't exist" in the traditional sense; beings are anatta. But we are certainly alive -- we are not imagination.