  The metastable Helium (He*) lines near $10830~{\rm A}$ are ideal probes of
atmospheric erosion--a common phenomenon of close-in exoplanet evolution. A
handful of exoplanet observations yielded well-resolved He* absorption features
in transits, yet they were mostly analyzed with 1D isothermal models
prescribing mass-loss rates. This work devises 3D hydrodynamics co-evolved with
ray-tracing radiative transfer and non-equilibrium thermochemsitry. Starting
from the observed stellar/planetary properties with reasonable assumptions
about the host's high energy irradiation, we predict from first principle the
mass loss rate, the temperature and ionization profiles, and 3D outflow
kinematics. Our simulations well reproduce the observed He* line profiles and
light curves of WASP-69b. We further investigate the dependence of He*
observables on simulation conditions and host radiation. The key findings are:
(1) Simulations reveal a photoevaporative outflow ($\sim 0.5~M_{\oplus}~{\rm
Gyr}^{-1}$) for WASP-69b without a prominent comet-like tail, consistent with
the symmetric transit shape (Vissapragada et al. 2020). (2) 3D simulations are
mandatory for hydrodynamic features, including Coriolis force, advection, and
kinematic line broadening. (3) EUV ($>13.6~{\rm eV}$) photons dominate
photoevaporative outflows and populate He* via recombination; FUV is also
detrimental by destroying He*; X-ray plays a secondary role. (4) K stars hit
the sweet spot of EUV/FUV balance for He* line observation, while G and M stars
are also worthy targets. (5) Stellar flars create characteristic responses in
the He* line profiles.
