Yaml files created with yaml and yml file extension, Both are the same in interpretation and syntax.

In Older versions of windows, Extensions restricts to 3 letters like .yml

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.