  This paper presents simulations of the metastable helium (He*) observations
of WASP-107b, so far the highest signal-to-noise ratio detection that is
confirmed by three different instruments. We employ full 3D hydrodynamics
coupled with co-evolving non-equilibrium thermochemistry and ray-tracing
radiation, predicting mass loss rates, temperature profiles, and synthetic He*
line profiles and light curves from first principles. We found that a stellar
wind stronger than solar is demanded by the observed heavily blueshifted line
profile and asymmetric transit light curve. Contrary to previous beliefs, we
argue that radiation pressure can be important for Ly$\alpha$ observations but
{\it not} He*. We found WASP-107b is losing mass at a rate of $\dot{M} \simeq
1.0\times 10^{-9}~M_\oplus~{\rm yr}^{-1}$. Although $\dot{M}$ varies by $<1~\%$
given constant wind and irradiation from the host, shear instabilities still
emerge from wind impacts, producing $\sim 10~\%$ fluctuations of He* transit
depths over hour-long timescales. The common assumption that He* transit depth
indicates the fluctuation of $\dot{M}$ is problematic. The trailing tail is
more susceptible than planet adjacency to the shear instabilities, thus the
line profile is more variable in the blue-shifted wing, while the transit light
curve is more variable after mid-transit. We stress the synergy between
Ly$\alpha$ (higher altitudes, lower density) and He* (lower altitudes, higher
density) transit observations, particularly simultaneous ones, yield better
understanding of planetary outflows and stellar wind properties.
