I know next to nothing about physics so forgive me for this question.
What are the exact mechanics behind gravity and how does it work?
The current description of gravity is General Relativity. In a nutshell mass and energy bends spacetime and objects just move along "straight lines" (actually geodesics) in this curved spacetime, which is percieved as an accelerated motion for an external observer.
Until now there has been no evidence against this description, although some predictions still await direct confirmation (gravity waved for example)
Are there nuclear particles that determine mass or some other force?
A lot of mass in our world is actually just binding energy between sub-nuclear particles. The mass of the proton is about 100 times the mass of its constituent quarks, the rest is binding energy, which effectively acts as mass.
However, (most) sub-nuclear particles do have mass, which is created by coupling to the Higgs field. The gauge boson of this field, the Higgs boson is indeed some sort of particle that determines the mass of other particles (and itself).
The problem with these descriptions of mass is that the underlying theories have to ignore gravity, so the connection between this mass and the mass relevant for gravity is not exactly clear.
I thought i heard somewhere that scientists don't exactly know how gravity worked but i thought they knew some specifics about it.
With GR we do have a theory of gravity that has not been falsified in any way, yet. What we do not have is a quantum theory of gravity, as all attempts to unify GR and quantum field theory have either failed or do not give any testable predictions. So we have no idea what happens, when quantum gravity effects become relevant. But experimentally we are a long way from realizing such conditions.
So in other words: We can explain every gravity effect we can see, but we have no idea what happens in very extreme conditions.
(Note that the explanations can only go to a certain level. At one point there will always be formulas, constants or assumptions with no explanation why these things are what they are)