  We investigate the formation of jellyfish galaxies using
radiation-hydrodynamic simulations of gas-rich dwarf galaxies with a
multi-phase interstellar medium (ISM). We find that the ram-pressure-stripped
(RPS) ISM is the dominant source of molecular clumps in the near wake within 10
kpc from the galactic plane, while in-situ formation is the major channel for
dense gas in the distant tail of the gas-rich galaxy. Only 20% of the molecular
clumps in the near wake originate from the intracluster medium (ICM); however,
the fraction reaches 50% in the clumps located at $80\,{\rm kpc}$ from the
galactic center since the cooling time of the RPS gas tends to be short due to
the ISM--ICM mixing ($\lesssim$ 10 Myr). The tail region exhibits a star
formation rate of $0.001-0.01\,{\rm M_{\odot}\,yr^{-1}}$, and most of the tail
stars are born in the stripped wake within 10 kpc from the galactic plane.
These stars induce bright H$\alpha$ blobs in the tail, while H$\alpha$ tails
fainter than $6\times10^{38}\,{\rm erg\,s^{-1}\,kpc^{-2}}$ are mostly formed
via collisional radiation and heating due to mixing. We also find that the
stripped tails have intermediate X-ray to H$\alpha$ surface brightness ratios
(1.5$\lesssim F_{\rm X}/F_{\rm H\alpha}\lesssim$20), compared to the ISM
($\lesssim$1.5) or pure ICM ($\gg$20). Our results suggest that jellyfish
features emerge when the ISM from gas-rich galaxies is stripped by strong ram
pressure, mixes with the ICM, and enhances the cooling in the tail.
