  Using advanced dynamical mean-field theory on a realistic level we study the
normal-state correlated electronic structure of the high-pressure
superconductor La$_3$Ni$_2$O$_7$ and compare the features of the conventional
bilayer (2222) Ruddelsden-Popper crystal structure with those of a
newly-identified monolayer-trilayer (1313) alternation. Both structural cases
display Ni-$d_{z^2}$ flat-band character at low-energy, which drives an
electronic instability with a wave vector ${\bf q_{\rm I}}=(0.25,0.25,q_z)$ at
ambient pressure, in line with recent experimental findings. The 1313
electronic structure exhibits significant layer selectivity, rendering
especially the monolayer part to be Mott-critical. At high pressure, this layer
selectivity weakens and the 1313 fermiology displays arcs reminiscent to those
of high-$T_c$ cuprates. In contrast to dominant inter-site self-energy effects
in the latter systems, here the Fermi arcs are the result of the multiorbital
and multilayer interplay within a correlated flat-band scenario.
