  Through tracing back to EA/SK models, we disentangle the construction logic
of SYK model. During the construction, we recall the technical essence of
replica method. Then we point out the redundance of the flavor group and the
slave indices as well as the color group via the generalization from 2-index SY
model to its 4-index version and the simplification of the latter in terms of
Majorana fermions. Spectacularly, such a simplification reserves the system's
self-consistency condition. Getting rid of the redundance, the model itself and
its effective action have the same $O(N_s)$ structural symmetry, while it
realizes $AdS_2/CFT_1$ holography by the emergent conformal symmetry. We also
clarify the model transmutation to matrix model and argue that the
disorder-averaged SYK model naturally preserves a holographic nature. Based on
the transmutation and the correspondence with vector model, SYK model is
formally a hybrid of these two models. Its quantum and semiclassical properties
are also discussed respectively.
