  We analyze the physical properties of gas in the circumgalactic medium (CGM)
of 132 Milky Way (MW)-like central galaxies at $z=0$ from the cosmological
magneto-hydrodynamical simulation TNG50, part of the IllustrisTNG project. The
properties and abundance of CGM gas across the sample are diverse, and the
fractional budgets of different phases (cold, warm, and hot), as well as
neutral HI mass and metal mass, vary considerably. Over our stellar mass range
of $10^{10.5} < M_\star / \rm{M}_\odot < 10^{10.9}$, radial profiles of gas
physical properties from $0.15 < R\rm{ / R_{\rm 200c}} < 1.0$ reveal great CGM
structural complexity, with significant variations both at fixed distance
around individual galaxies, and across different galaxies. CGM gas is
multi-phase: the distributions of density, temperature and entropy are all
multimodal, while metallicity and thermal pressure distributions are unimodal;
all are broad. We present predictions for magnetic fields in MW-like halos: a
median field strength of $|B|\sim\,1\mu$G in the inner halo decreases rapidly
at larger distance, while magnetic pressure dominates over thermal pressure
only within $\sim0.2 \times \rm{R_{200c}}$. Virial temperature gas at $\sim
10^6\,$K coexists with a sub-dominant cool, $< 10^5\,$K component in
approximate pressure equilibrium. Finally, the physical properties of the CGM
are tightly connected to the galactic star formation rate, in turn dependent on
feedback from supermassive black holes (SMBHs). In TNG50, we find that energy
from SMBH-driven kinetic winds generates high-velocity outflows ($\gtrsim
500-2000$ km/s), heats gas to super-virial temperatures ($> 10^{6.5-7}$ K), and
regulates the net balance of inflows versus outflows in otherwise quasi-static
gaseous halos.
