yamlfmt can collect paths in two modes: Standard, or Doublestar.
In standard path mode, the you can specify a file or directory path directly. If specifying a file, it will simply include the file. If specifying a directory, it will include every file with the correct extension (as specified in extensions, default is yml and yaml).
In Doublestar mode, paths are specified using the format explained in the doublestar package. It is almost identical to bash and git's style of glob pattern specification.