  The incommensurate 30$^{\circ}$ twisted bilayer graphene (BG) possesses both
relativistic Dirac fermions and quasiperiodicity with 12-fold rotational
symmetry arising from the interlayer interaction
[\href{https://science.sciencemag.org/content/361/6404/782}{Ahn et al., Science
\textbf{361}, 782 (2018)} and
\href{https://www.pnas.org/content/115/27/6928}{Yao et al., Proc. Natl. Acad.
Sci. \textbf{115}, 6928 (2018)}]. Understanding how the interlayer states
interact with each other is of vital importance for identifying and
subsequently engineering the quasicrystalline order in the layered structures.
Herein, via symmetry and group representation theory we unravel the interlayer
hybridization selection rules governing the interlayer coupling in both
untwisted and twisted BG systems. Compared with the only allowed equivalent
hybridization in $D_{6h}$ untwisted BG, $D_6$ twisted BG permits equivalent and
mixed hybridizations, and $D_{6d}$ graphene quasicrystal allows both equivalent
and non-equivalent hybridizations. The energy-dependent hybridization strengths
in graphene quasicrystal and $D_6$ twisted BG show two remarkable
characteristics: (i) near the Fermi level the weak hybridization owing to the
relatively large energy difference between Dirac bands from top and bottom
layers, and (ii) in high-energy regions the electron-hole asymmetry of
hybridization strength with stronger interlayer coupling for holes, which
arises from the non-nearest-neighbor interlayer hoppings and the wave-function
phase difference between paring states. These hybridization-generated band
structures and their hybridization strength characteristics are verified by the
calculated optical conductivity spectra. Our theoretical study paves a way for
revealing the interlayer hybridization in van der Waals layered systems.
