  The detection of the hyper-bright gamma-ray burst (GRB) 221009A enables us to
explore the nature of GRB emission and the origin of very-high-energy (VHE)
gamma-rays. We analyze the ${\it Fermi}$-LAT data and investigate GeV-TeV
emission in the framework of the external reverse shock model. We show that
early $\sim1-10$ GeV emission can be explained by the external inverse-Compton
mechanism via upscattering MeV gamma-rays by electrons accelerated at the
reverse shock, in addition to the synchrotron self-Compton component. The
predicted early optical flux could have been brighter than the naked-eye GRB
080319B. We also show that proton synchrotron emission from accelerated
ultra-high-energy cosmic rays (UHECRs) is detectable, and could potentially
explain $\gtrsim \rm TeV$ photons detected by LHAASO or UHECR acceleration can
be constrained. Our model suggests that the detection of
$\mathcal{O}(10\rm~TeV)$ photons with energy up to $\sim18$ TeV is possible for
reasonable models of the extragalactic background light without invoking new
physics, and predicts anti-correlations between MeV photons and TeV photons,
which can be tested with the LHAASO data.
