Belle II is an experiment designed to study billions of $\tau$-lepton, $b$- and $c$-quark decays observed with low background in asymmetric-energy electron-positron collisions at the SuperKEKB $B$-factory. In March 2019, the newly completed Belle II started operating and collected its first physics data reaching 10 fb$^{-1}$ to date. We report the reconstruction of prominent signals from various hadronic $B$ decays including $B^{-}\to D^{(*)0}\pi^{-}$, $B^{0}\to D^{-}K^{+}$, and $B^{0}\to K^{+}\pi^{-}$ in the first data set corresponding to 5.15 fb$^{-1}$. These results show a remarkable level of early understanding of detector performance.