concat returns a lazy sequence.
user=> (doc concat)
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clojure.core/concat
([] [x] [x y] [x y & zs])
Returns a lazy seq representing the concatenation of the elements in the supplied colls.
you can convert it back to a vector with into:
user=> (into [] (concat [1 2] [3 4] [5 6]))
[1 2 3 4 5 6]
into uses transients so it’s pretty quick about it.