  At magic twisted angles, Dirac cones in twisted bilayer graphene (TBG) can
evolve into flat bands, serving as a critical playground for the study of
strongly correlated physics. When chiral symmetry is introduced, rigorous
mathematical proof confirms that the flat bands are locked at zero energy in
the entire Moir\'e Brillouin zone (BZ). Yet, TBG is not the sole platform that
exhibits this absolute band flatness. Central to this flatness phenomenon are
topological nodes and their specific locations in the BZ. In this study,
considering twisted bilayer systems that preserve chiral symmetry, we classify
various ordered topological nodes in base layers and all possible node
locations across different BZs. Specifically, we constrain the node locations
to rotational centers, such as {\Gamma} and M points, to ensure the interlayer
coupling retains equal strength in all directions. Using this classification as
a foundation, we systematically identify the conditions under which Moir\'e
flat bands emerge. Additionally, through the extension of holomorphic
functions, we provide proof that flat bands are locked at zero energy, shedding
light on the origin of the band flatness. Remarkably, beyond Dirac cones,
numerous twisted bilayer nodal platforms can host flat bands with a degeneracy
number of more than two, such as four-fold, six-fold, and eight-fold. This
multiplicity of degeneracy in flat bands might unveil more complex and enriched
correlation physics.
