  High-energy neutrino and $\gamma$-ray emission has been observed from the
Galactic plane, which may come from individual sources and/or diffuse cosmic
rays. We evaluate the contribution of these two components through the
multimessenger connection between neutrinos and $\gamma$ rays in hadronic
interactions. We derive maximum fluxes of neutrino emission from the Galactic
plane using $\gamma$-ray catalogs, including 4FGL, HGPS, 3HWC, and 1LHAASO, and
measurements of the Galactic diffuse emission by Tibet AS$\gamma$ and LHAASO.
We find that the IceCube Galactic neutrino flux is larger than the contribution
from all resolved sources when excluding promising leptonic sources such as
pulsars, pulsar wind nebulae, and TeV halos. Our result indicates that the
Galactic neutrino emission is likely dominated by the diffuse emission by the
cosmic-ray sea and unresolved hadronic $\gamma$-ray sources. In addition, the
IceCube flux is comparable to the sum of the flux of non-pulsar sources and the
LHAASO diffuse emission especially above 30 TeV. This implies that the LHAASO
diffuse emission may dominantly originate from hadronic interactions, either as
the truly diffuse emission or unresolved hadronic emitters. Future observations
of neutrino telescopes and air-shower $\gamma$-ray experiments in the Southern
hemisphere are needed to accurately disentangle the source and diffuse emission
of the Milky Way.
