We calculate the $B\!\to\!D^{(*)}\ell\nu$ form factors in 2+1 flavor
relativistic lattice QCD by employing the Moebius domain-wall action
for all quark flavors.
Our simulations are carried out at lattice cut-offs $a^{-1} \sim 2.5$,
3.6 and 4.5 GeV with the bottom quark masses up to 0.7 $a^{-1}$
to control discretization effects.
We extrapolate the form factors to the continuum limit and
physical quark masses, and discuss systematic uncertainties
of the form factors.