  In this paper, we present high-resolution spectroscopic transit observations
from ESPRESSO of the super-Neptune WASP-166~b. In addition to spectroscopic
ESPRESSO data, we analyse photometric data from {\sl TESS} of six WASP-166~b
transits along with simultaneous NGTS observations of the ESPRESSO runs. These
observations were used to fit for the planetary parameters as well as assessing
the level of stellar activity (e.g. spot crossings, flares) present during the
ESPRESSO observations. We utilise the Reloaded Rossiter McLaughlin (RRM)
technique to spatially resolve the stellar surface, characterising the
centre-to-limb convection-induced variations, and to refine the star-planet
obliquity. We find WASP-166~b has a projected obliquity of $\lambda =
-15.52^{+2.85}_{-2.76}$$^{\circ}$ and $v\sin(i) = 4.97 \pm 0.09$~kms$^{-1}$
which is consistent with the literature. We were able to characterise
centre-to-limb convective variations as a result of granulation on the surface
of the star on the order of a few kms$^{-1}$ for the first time. We modelled
the centre-to-limb convective variations using a linear, quadratic and cubic
model with the cubic being preferred. In addition, by modelling the
differential rotation and centre-to-limb convective variations simultaneously
we were able to retrieve a potential anti-solar differential rotational shear
($\alpha \sim$ -0.5) and stellar inclination ($i_*$ either
42.03$^{+9.13}_{-9.60}$$^{\circ}$ or 133.64$^{+8.42}_{-7.98}$$^{\circ}$ if the
star is pointing towards or away from us). Finally, we investigate how the
shape of the cross-correlation functions change as a function of limb angle and
compare our results to magnetohydrodynamic simulations.
