Do you have any guidelines for deciding which activities/process tasks should go to level 1 or level 2 or even beyond?
It is really specific to the process task being mapped. From a general standpoint, level 1 begins to show some of the details with decisions and various flow paths. Level 2 walks through steps at a level that someone new to the process would need to know.
What if there are level 3 and beyond? How should the numbering convention be like?
There actually isn't a level 3 and beyond. Level 2 is the most detail shown within a process flowchart.
Is the stored data symbol to replace the process task symbol (thought this is what is mentioned in L46 video), or it comes after the process task to indicate where/what data is stored?
It really depends on how you are using the data. If you need to transform the data before you store it (for example, converting from one currency to another), you will need a process task to do that before it hits the data symbol. If you are simply storing the data received in the previous step, then a stored data symbol will only be needed.
Can you please elaborate technical user story? Steps to write it? What should be the length of user story? Do you have any template for that? how detailed it should be?
For what I have done, my technical user stories generally follow the same flow and principle as a functional user story. I still like to use the [As a]... [I need to]... [so]... format.
When I am not writing them like that I instead write it out like a normal requirement. For example, "The system needs to authenticate user access every 5 minutes." Then the acceptance criteria would go into the specific details necessary.
From a length standpoint, I like 1-2 sentences, but as long as you keep it concise and to the point it should be okay.