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