  With a quark level interaction, we give a unified description of the loosely
bound molecular systems composed of the heavy flavor hadrons
$(\bar{D},\bar{D}^*)$, $(\Lambda_c, \Sigma_c, \Sigma_c^*)$, and $(\Xi_c,
\Xi_c^\prime,\Xi_c^*)$. Using the $P_c$ states as inputs to fix the interaction
strength of light quark-quark pairs, we reproduce the observed $P_{cs}$ and
$T_{cc}^+$ states and predict another narrow $T_{cc}^{\prime+}$ state with
quantum numbers $[D^*D^*]_{J=1}^{I=0}$. If we require a satisfactory
description of the $T_{cc}^+$ and $P_c$ states simultaneously, our framework
prefers the assignments of the $P_{c}(4440)$ and $P_{c}(4457)$ as the
$[\Sigma_c\bar{D}^*]_{J=1/2}^{I=1/2}$ and $[\Sigma_c\bar{D}^*]_{J=3/2}^{I=1/2}$
states, respectively. We propose the isospin criterion to explain naturally why
the experimentally observed $T_{cc}$, $P_c$, and $P_{cs}$ molecular candidates
prefer the lowest isospin numbers. We also predict the loosely bound states for
the bottom di-hadrons.
