In Lisp you'd do something like

    (let* ((my-list '(1 2 3))
           (my-other-list '(4 5 6))
           (final-list (concatenate 'list my-list my-other-list)))
       (whatever-cl-calls-join final-list ""))

This is a single Lisp form evaluated at the REPL that you could get back and edit as necessary. It's been a decade or so haha but that's the idea