...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 M-x 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*You*Need > 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.

...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 M-x 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.