  The exotic singlet fermions $\chi$, with a mass $m_\chi\lesssim 50$ MeV,
could be produced at the coherent elastic neutrino-nucleus scattering
(CE$\nu$NS) experiments through the $\nu {\mathcal N} \rightarrow \chi
{\mathcal N}$ process. Due to the coherent enhancement, it offers a unique way
to study how $\chi$ interacts with the Standard Model (SM) sector. Based on the
most general dimension-6 effective Lagrangian, we perform a comprehensive study
on the relevant interaction between $\chi$ and the SM sector. From the current
and future COHERENT and future CONUS experiments, we obtain the upper bounds on
the Wilson coefficients for the dipole, scalar, vector, and tensor
interactions. For $m_\chi $ below 10 MeV, future CONUS data has the best
sensitivity, while for $m_\chi$ between 10 MeV$-50$ MeV, the current and future
COHERENT bounds dominate. These limits are complementary to those from neutrino
oscillation and collider searches. Moreover, the bounds do not depend on the
charge conjugation property of $\chi$, nor whether $\chi$ is dark matter or
not.
