  We report our theoretical calculations on the branching fractions for the
semileptonic $B$ and $B_s$ decays, i.e., $B (B_s) \to (P,\, V,\, S,\,A) \ell
\nu_\ell$, where $P$ and $V$ denote the pseudoscalar and vector mesons,
respectively, while $S$ denotes the scalar meson with mass above 1 GeV and $A$
the axial-vector meson. The branching fractions for the semileptonic $B\to P$
and $V$ modes have been measured very well in experiment and our theoretical
values are in good agreement with them. The ones for $B\to S$ and $A$ modes are
our theoretical predictions. There is little experimental information on the
semileptonic $B_s$ decays although much theoretical effort has been done. In
addition, we predict the branching fractions of $B\to D^*_0(2400) \ell
\bar\nu_\ell$ and $B_s\to D^{*-}_{s0}(2317) \ell \bar\nu_\ell$ as $(2.31\pm
0.25)\times 10^{-3}$ and $(3.07\pm0.34)\times 10^{-3}$, in order, assuming them
as the conventional mesons with quark-antiquark configuration. The high
luminosity $e^+e^-$ collider SuperKEKB/Belle-II is running, with the data
sample enhanced by a factor of 40 compared to Belle, which will provide huge
opportunity for the test of the theoretical predictions and further help
understand the inner structure of these scalar and axial vector mesons, e.g.,
the glueball content of $f_0(1710)$ and the mixing angles for the axial-vector
mesons. These decay channels can also be accessed by the LHCb experiment.
