As I understand, NPC set contains only the problems which can be polynomially converted into each other and which are hardest in NP set/ But how do we decide which problems are in NPC and which problems are not in NPC but only in NP (not even in P)?
Or there are no such problems which are in NP, but not in P or NPC?
Define $NPI = NP \setminus NPC \setminus P$, the $I$ stands for Intermediate. If $P = NP$ then $NPI = \emptyset$. The converse is also true, this is Ladner's theorem. Many problems are suspected to be in $NPI$, assuming $P \ne NP$, most importantly factoring and graph isomorphism. Since we can't even prove $NPI$ is non-empty, any problem that is known to be in $NP$ but not known to be in $P$ and not known to be in $NPC$ is possibly in $NPI$ and may even be suspected to be, much like real numbers not known to be algebraic are suspected to be transcendental.