  JWST observations confirm the existence of galaxies as early as 300Myr and at
a higher number density than expected based on galaxy formation models and HST
observations. Yet, sources confirmed spectroscopically in the first 500Myr have
estimated stellar masses $<5\times10^8M_\odot$, limiting the signal to noise
ratio (SNR) for investigating substructure. We present a high-resolution
spectroscopic and spatially resolved study of a rare bright galaxy at
$z=9.3127\pm0.0002$ with a stellar mass of
$(2.5^{+0.7}_{-0.5})\times10^9M_\odot$, forming $25^{+3}_{-4}M_\odot/yr$ and
with a metallicity of $\sim0.1Z_\odot$- lower than in the local universe for
the stellar mass but in line with expectations of chemical enrichment in
galaxies 1-2Gyr after the Big Bang. The system has a morphology typically
associated to two interacting galaxies, with a two-component main clump of very
young stars (age$<10$Myr) surrounded by an extended stellar population
($130\pm20$Myr old, identified by modeling the NIRSpec spectrum) and an
elongated clumpy tidal tail. The spectroscopic observations identify O, Ne and
H emission lines, and the Lyman break, where there is evidence of substantial
Ly$\alpha$ absorption. The [OII] doublet is resolved spectrally, enabling an
estimate of the electron number density and ionization parameter of the
interstellar medium and showing higher densities and ionization than in lower
redshift analogs. For the first time at $z>8$, we identify evidence of
absorption lines (Si, C and Fe), with low confidence individual detections but
SNR$>6$ when stacked. The absorption features suggest that Ly$\alpha$ is damped
by the interstellar and circumgalactic medium. Our observations provide
evidence of rapid efficient build-up of mass and metals in the immediate
aftermath of the Big Bang through mergers, demonstrating that massive galaxies
with several billion stars exist earlier than expected.
