  We model the newly synthesized magic-angle twisted bilayer-graphene
superconductor with two $p_{x,y}$-like Wannier orbitals on the superstructure
honeycomb lattice, where the hopping integrals are constructed via the
Slater-Koster formulism by symmetry analysis. The characteristics exhibited in
this simple model are well consistent with both the rigorous calculations and
experiment observations. A van Hove singularity and Fermi-surface (FS) nesting
are found in the doping levels relevant to the correlated insulator and
unconventional superconductivity revealed experimentally, base on which we
identify the two phases as weak-coupling FS instabilities. Then, with repulsive
Hubbard interactions turning on, we performed random-phase-approximation (RPA)
based calculations to identify the electron instabilities. As a result, we find
chiral $d+id$ topological superconductivity bordering the correlated insulating
state near half-filling, identified as noncoplanar chiral spin-density wave
(SDW) ordered state, featuring quantum anomalous Hall effect. The phase-diagram
obtained in our approach is qualitatively consistent with experiments.
