See this out of the box functionality documented here:
https://docs.adobe.com/docs/en/aem/6-1/author/assets/dynamic-media/video.html#Publishing videos to YouTube
If this does not meet your business requirements and you want to customize it (ie - build a workflow step) - then you need to look at using Google YouTube APIs and build an AEM service.
There is a Java code example that shows you how to upload a youtube video here:
https://developers.google.com/youtube/v3/code_samples/java?hl=en#upload_a_video
You need to write an OSGi AEM service that uses code shown in the above code example. Also - to make this work - even in a Java standalone app in main (in Eclipse) - you need to follow the Google docs and setup OAuth credentials.
This is explained here:
https://developers.google.com/youtube/registering_an_application
Then you need to reference all the Google APIs in the POM file of the Maven project that you use to build a AEM service.
Finally - you need to wrap all of the Google APIs in an OSGi bundle and deploy to AEM. All of these Google JAR files need to be part of the OSGi service container.
If you do not know how to build a custom AEM service (an OSGi bundle) using Java and Maven - see this article to teach you how to get up and running.
https://helpx.adobe.com/experience-manager/using/first-osgi.html
Before you even try building this service - I would strongly recommend that you setup a standalone Java Eclipse app that runs this Google UploadVideo Code in main(). Make sure you setup the credentials and everything works. Once you get this far - you can try and start porting the code to an AEM service.
