  We generalize the framework of chiral effective field theory to study the
interactions of the isovector $D^\ast\bar{D}^{(\ast)}$ and
$B^\ast\bar{B}^{(\ast)}$ systems up to the next-to-leading order, in which the
long-, mid-, and short-range force contributions as well as the $S$-$D$ wave
mixing are incorporated. Based on the Lippmann-Schwinger equation, we fit the
invariant mass distributions of the elastic channels measured by the BESIII and
Belle Collaborations. Our results indicate that the four charged charmoniumlike
and bottomoniumlike states $Z_c(3900)$, $Z_c(4020)$ and $Z_b(10610)$,
$Z_b(10650)$ can be well identified as the
$D\bar{D}^{\ast},D^\ast\bar{D}^{\ast}$ and
$B\bar{B}^{\ast},B^\ast\bar{B}^{\ast}$ molecular resonances. The bound state
explanations are vetoed in our framework. Our study favors the $Z_c$ and $Z_b$
states are the twin partners under the heavy quark symmetry.
