What Silverstripe Learning Resources available over the net?
A good place to start is here: https://www.silverstripe.org/learn/
There are links to the API documentation, developer docs, step-by-step lessons, the Slack channel, etc.
This forum is also a useful resource if you have specific questions.
The link is a bit hidden at the bottom 
@Tim is there something very simple to follow like a tutorial ? How to Publish a SilverStripe module – SilverStripe Documentation is about publishing the module but what is the workflow step by step in creating an actual module?
There are more notes about modules here: Modules – SilverStripe Documentation
In terms of describing the workflow for creating a module, I don’t know of a specific step-by-step guide at the moment. The how-to and the link above should describe how the module code needs to be organised. What that code does is obviously very much up to you and what you want the module to do.
If there are features that you think should be added (such as specific tutorials), it might be worth posting a suggestion here: New Features: Hot (95 ideas) – Community Feeback for SilverStripe CMS & Framework where it can be logged and considered.
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Just a follow-up for anyone who might be reading, there is a video tutorial by the venerable @unclecheese here: https://www.silverstripe.org/learn/lessons/v3/publishing-your-own-module
This is based on SS3, so there will be some differences in the way you approach a module for SS4 which you should be able to identify from the links in this thread, but the overall process is still valid.