On the storybook website, they describe customizing your configuration using files named preview.js and main.js.
At the company I work at, it seems like the .storybook directory seems to contain a bunch of files with other names, like a webpack.config.js or even just config.js.
Does it not matter what you name the files in the .storybook directory? Does it just run all of them arbitrarily?
The file names very much matter. What is happening is that your project is using the older standard (though it's still compatible) and the file names have been changed since Storybook 5.3. I understand it can be confusing, but the information available in the migration guide should help.