...And then there is the Eshell-way. It uses Elisp under the hood when needed. Therefore it is OS-independent:

1. Open an Eshell via <kbd>M-x</kbd> `eshell`.

2. Change directory in the Eshell to the one you want with `cd YourDirectoryPath`.

3. Concatenate all files you want in Eshell with `cat Pattern*Of*Source*Files > targetFile`.


Note, `emacs -Q` uses Elisp for all above steps under Ubuntu (WSL):

    ~ $ which cd
    eshell/cd is a compiled Lisp function in ‘em-dirs.el’.
    ~ $ which cat
    eshell/cat is a compiled Lisp function in em-unix.el.