  Using a representative sample of Milky Way (MW)-like galaxies from the TNG50
cosmological simulation, we investigate physical processes driving the
formation of galactic disks. A disk forms as a result of the interplay between
inflow and outflow carrying angular momentum in and out of the galaxy.
Interestingly, the inflow and outflow have remarkably similar distributions of
angular momentum, suggesting an exchange of angular momentum and/or outflow
recycling, leading to continuous feeding of prealigned material from the
corotating circumgalactic medium. We show that the disk formation in TNG50 is
correlated with stellar bulge formation, in qualitative agreement with a recent
theoretical model of disk formation facilitated by steep gravitational
potentials. Disk formation is also correlated with the formation of a hot
circumgalactic halo with around half of the inflow occurring at subsonic and
transonic velocities corresponding to Mach numbers of $\lesssim2$. In the
context of recent theoretical works connecting disk settling and hot halo
formation, our results imply that the subsonic part of the inflow may settle
into a disk while the remaining supersonic inflow will perturb this disk via
the chaotic cold accretion. We find that disks tend to form when the host halos
become more massive than $\sim (1-2) \times 10^{11} M_\odot$, consistent with
previous theoretical findings and observational estimates of the predisk
protogalaxy remnant in the MW. Our results do not prove that either corotating
outflow recycling, gravitational potential steepening, or hot halo formation
cause disk formation, but they show that all these processes occur concurrently
and may play an important role in disk growth.
