  High-redshift star-forming galaxies have very different morphologies compared
to nearby ones. Indeed, they are often dominated by bright star-forming
structures of masses up to $10^{8-9}$ $\mathrm{M}_\odot$ dubbed
{\guillemotleft}giant clumps{\guillemotright}. However, recent observations
questioned this result by showing only low-mass structures or no structure at
all. We use Adaptative Mesh Refinement hydrodynamical simulations of galaxies
with parsec-scale resolution to study the formation of structures inside clumpy
high-redshift galaxies. We show that in very gas-rich galaxies star formation
occurs in small gas clusters with masses below $10^{7-8}$ $\mathrm{M}_\odot$
that are themselves located inside giant complexes with masses up to $10^8$ and
sometimes $10^9$ $\mathrm{M}_\odot$. Those massive structures are similar in
mass and size to the giant clumps observed in imaging surveys, in particular
with the Hubble Space Telescope. Using mock observations of simulated galaxies,
we show that at very high resolution with instruments like the Atacama Large
Millimeter Array or through gravitational lensing, only low-mass structures are
likely to be detected, and their gathering into giant complexes might be
missed. This leads to the non-detection of the giant clumps and therefore
introduces a bias in the detection of these structures. We show that the
simulated giant clumps can be gravitationally bound even when undetected in
mocks representative for ALMA observations and HST observations of lensed
galaxies. We then compare the top-down fragmentation of an initially warm disc
and the bottom-up fragmentation of an initially cold disc to show that the
process of formation of the clumps does not impact their physical properties.
