  A symmetry-preserving continuum approach to meson bound-states in quantum
field theory, employed elsewhere to describe numerous $\pi$- and $K$-meson
electroweak processes, is used to analyse leptonic and semileptonic decays of
$D_{(s)}$ mesons. Each semileptonic transition is conventionally characterised
by the value of the dominant form factor at $t=0$ and the following results are
obtained herein: $f_+^{D_s\to K}(0) = 0.673(40)$; $f_+^{D\to
\pi}(0)=0.618(31)$; and $f_+^{D\to K}(0)=0.756(36)$. Working with the computed
$t$-dependence of these form factors and standard averaged values for
$|V_{cd}|$, $|V_{cs}|$, one arrives at the following predictions for the
associated branching fractions: ${\cal B}_{D_s^+\to K^0 e^+ \nu_e} =
3.31(33)\times 10^{-3}$; ${\cal B}_{D^0\to \pi^- e^+ \nu_e} = 2.73(22)\times
10^{-3}$; and ${\cal B}_{D^0\to K^- e^+ \nu_e} = 3.83(28)$%. Alternatively,
using the calculated $t$-dependence, agreement with contemporary empirical
results for these branching fractions requires $|V_{cd}|=0.221(9)$,
$|V_{us}|=0.953(34)$. With all $D_{(s)}$ transition form factors in hand, the
nature of SU$(3)$-flavour symmetry-breaking in this array of processes can be
analysed; and just as in the $\pi$-$K$ sector, the magnitude of such effects is
found to be determined by the scales associated with emergent mass generation
in the Standard Model, not those originating with the Higgs mechanism.
