  We show that a tensor product of nonexceptional type Kirillov--Reshetikhin
(KR) crystals is isomorphic to a direct sum of Demazure crystals; we do this in
the mixed level case and without the perfectness assumption, thus generalizing
a result of Naoi. We use this result to show that, given two tensor products of
such KR crystals with the same maximal weight, after removing certain
$0$-arrows, the two connected components containing the minimal/maximal
elements are isomorphic. Based on the latter fact, we reduce a tensor product
of higher level perfect KR crystals to one of single-column KR crystals, which
allows us to use the uniform models available in the literature in the latter
case. We also use our results to give a combinatorial interpretation of the
Q-system relations. Our results are conjectured to extend to the exceptional
types.
