  GRB~230812B is a bright and relatively nearby ($z =0.36$) long gamma-ray
burst (GRB) that has generated significant interest in the community and has
thus been observed over the entire electromagnetic spectrum. We report over 80
observations in X-ray, ultraviolet, optical, infrared, and sub-millimeter bands
from the GRANDMA (Global Rapid Advanced Network for Multi-messenger Addicts)
network of observatories and from observational partners. Adding complementary
data from the literature, we then derive essential physical parameters
associated with the ejecta and external properties (i.e. the geometry and
environment) of the GRB and compare with other analyses of this event. We
spectroscopically confirm the presence of an associated supernova, SN2023pel,
and we derive a photospheric expansion velocity of v $\sim$ 17$\times10^3$ km
s$^{-1}$. We analyze the photometric data first using empirical fits of the
flux and then with full Bayesian Inference. We again strongly establish the
presence of a supernova in the data, with a maximum (pseudo-)bolometric
luminosity of $5.75 \times 10^{42}$ erg/s, at $15.76^{+0.81}_{-1.21}$ days (in
the observer frame) after the trigger, with a half-max time width of 22.0 days.
We compare these values with those of SN1998bw, SN2006aj, and SN2013dx. Our
best-fit model favours a very low density environment ($\log_{10}({n_{\rm
ISM}/{\rm cm}^{-3}}) = -2.38^{+1.45}_{-1.60}$) and small values for the jet's
core angle $\theta_{\rm core} = 1.54^{+1.02}_{-0.81} \ \rm{deg}$ and viewing
angle $\theta_{\rm obs} = 0.76^{+1.29}_{-0.76} \ \rm{deg}$. GRB 230812B is thus
one of the best observed afterglows with a distinctive supernova bump.
