You can add that directory yourself. You’ll also want to ensure that if you went through the normal Rails upgrade path, using the task
rake app:update to update all your defaults, that the other parts of that tooling are also present and being loaded. To take full advantage of that system, you’ll need to have the webpacker gem installed and configured with its YAML file.
One of the easiest ways to see what is different between an application that was started with 6.x and one that was upgraded to that level is to spin up a new empty app with Rails 6, and then compare the preferences and defaults.