  We used dedicated SRG/eROSITA X-ray, ASKAP/EMU radio, and DECam optical
observations of a 15 sq.deg region around the interacting galaxy cluster system
A3391/95 to study the warm-hot gas in cluster outskirts and filaments, the
surrounding large-scale structure and its formation process. We relate the
observations to expectations from cosmological hydrodynamic simulations from
the Magneticum suite.
  We trace the irregular morphology of warm-hot gas of the main clusters from
their centers out to well beyond their characteristic radii, $r_{200}$. Between
the two main cluster systems, we observe an emission bridge; thanks to
eROSITA's unique soft response and large field of view, we discover tantalizing
hints for warm gas. Several matter clumps physically surrounding the system are
detected. For the "Northern Clump," we provide evidence that it is falling
towards A3391 from the hot gas morphology and radio lobe structure of its
central AGN. Many of the extended sources in the field detected by eROSITA are
known clusters or new clusters in the background, including a known SZ cluster
at redshift z=1. We discover an emission filament north of the virial radius,
$r_{100}$, of A3391 connecting to the Northern Clump and extending south of
A3395 towards another galaxy cluster. The total projected length of this
continuous warm-hot emission filament is 15 Mpc, running almost 4 degrees
across the entire eROSITA observation. The DECam galaxy density map shows
galaxy overdensities in the same regions. The new datasets provide impressive
confirmation of the theoretically expected structure formation processes on the
individual system level, including the surrounding warm-hot intergalactic
medium distribution compared to the Magneticum simulation. Our spatially
resolved findings show that baryons indeed reside in large-scale warm-hot gas
filaments with a clumpy structure.
