Alternatively, if you want it materialized do one of:
julia> vv = [[1,2], [3,4]]
2-element Vector{Vector{Int64}}:
[1, 2]
[3, 4]
julia> vcat(vv...)
4-element Vector{Int64}:
1
2
3
4
julia> reduce(vcat, vv)
4-element Vector{Int64}:
1
2
3
4
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