It should be no different from Vim. Namely,
In nvim help :h vimrc describes how the associated file is (still) referred to as a "vimrc" (or "config file") and just below that (:h $MYVIMRC) you'll see how/when the environment variable $MYVIMRC is assigned, i.e. during startup nvim looks for config in four places (vs five ...
I have a file called ~/.config/nvim/after/ftplugin_after.vim
The correct path you're looking for is actually:
Note that ftplugin needs to be a directory, and that the *.vim file inside it must be named after the filetype of the files you would like to match.
The ftplugin scripts must always match a specific filetype, ...