List structure transformation

Flatten /@ Thread[data, List, {2}]

{{x, y, a}, {x, y, b}, {x, y, c}, {x, y, d}, {x, y, e}}

Also:

Append @@@ Tuples[{{#}, #2}] & @@ data

{{x, y, a}, {x, y, b}, {x, y, c}, {x, y, d}, {x, y, e}}

ClearAll[dat]
dat = {{x, y}, {a, b, c, d, e}}
Append[dat[[1]], #] & /@ dat[[2]]

{{x, y, a}, {x, y, b}, {x, y, c}, {x, y, d}, {x, y, e}}

Also this may offer some added advantages

Table[Join[dat[[1]], {dat[[2, i]]}], {i, Length[dat[[2]]]}]

Seems like a good place to use MapThread. Problem is that it wants both lists to be the same length. So use Table to make it so.

{p, q} = {{x, y}, {a, b, c, d, e}}; MapThread[Append, {Table[p,Length[q]], q}]

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