WordPress detects a file as plugin when there is a plugin header. So you can store multiple plugins in one directory, and they all will be recognized as different plugins.
Each file with at least /* Plugin Name: something */ is a plugin.
The reason is that WordPress scans all PHP files in the main directory of a plugin.
You can use any name for the plugin file. Avoid non-plugin files in the main directory. They just eat runtime. Put all other PHP files into sub-directories.