Yaml files are created with yaml and yml file extensions, Both are used to represent YAML files and both perform same in interpretation and syntax.
In Older versions of Windows, Extensions restricts to 3 letters like .yml.
Yaml is an official, descriptive and standard file extension. It is preferred extension, used by specification and new users.
.yml is shorter version, used in older window version. It is still widely used in existing applicaitons
Nowadays, there is no OS system-level enforcement to have 3 letters in extensions.
Most of the yaml users are using .yaml as their preferred choice.
Conclusion
As long as the content inside yaml and yml follows the yaml.org syntax.
There is no value in the extension of yaml or ‘yml`. Both perform the same in syntax and programming language interpretation.
Use as per your project requirement to have a consistency.