the crypttab.d thing is called a drop-in directory. linux will read files you make in these and overwrite the config files with the values in the drop-in files. it’s a way to add your own edits to config files without editing the default config files directly, so you don’t have to say manually copy crypttab to crypttab.old for a backup and instead always have a default config to fall back to if things go wrong by just commenting out your few changes in the drop-in file.