Assembler is a simple utility for macOS. It does one thing, and one thing only. It merges separate .txt files together.
It works by dragging a bunch of .txt files into the app (or selecting all files within a folder) and it converts those .txt files into one .txt file. (I should mention that it works with Fountain, Markdown, txt, csv, tex, css, and js file formats.)
Why would anyone need this, you might be asking? Assembler is useful for writers who use a text editor and a workflow where they have multiple chapters managed as individual .txt files in a folder.
Before assembling, files can be rearranged in the app, which helpfully also shows the first line of the text in the file. If your chapter file names are sequential 1.txt, 2.txt, 3.txt, etc, they automatically appear in order.