Scrum roles and responsibilities

Hi,

The scenario of the issue that the sprint backlog has one thing. That thing is to add a new tab on website and let user to choose one of payment types.

These types  are credit card and bank account. And the user has to choose only one of the payment types. We can create many subtask(add tab, fetch credit card info, fetch bank account info, save the choose of the payment type..etc). 

Issue 1: Who should open these subtask(jira) and assigne them? 

>> What does your Team and PO think about the approaches mentioned ? What stops anyone in Dev team to create the necessary agreed tasks ? 

As per scrum guide -

The Development Team modifies the Sprint Backlog throughout the Sprint, and the Sprint Backlog emerges during the Sprint. This emergence occurs as the Development Team works through the plan and learns more about the work needed to achieve the Sprint Goal. As new work is required, the Development Team adds it to the Sprint Backlog.

Issue 2:team member as a developer has a responsible to run test cases(unit test and simple sanity doing by developer)?

>> What does the scrum guide says about the development team's roles & responsibilities?

Issue 1: Who should open these subtask(jira) and assigne them?

The Development Team is fully responsible for the work that they due to accomplish the items in the Sprint Backlog.  Therefore, Development Team is responsible for the tasking.  Who in the development team?  That is really up the team to decide.  What my teams tend to do is have 1 person write up the tasks for each story.  Then as a team, they review the tasks to agree/ensure that everything has been thought of and captured.

As for assigning them, why should someone assign them in a self-managed, self-organized team.  Can't the person with the skills and time available just pull that item into the "In progress" state and assign it to themselves? Why does someone have to be "in charge"?

One thing I suggest to my teams is that you create tasks at the appropriate level. If it takes you longer to write up a set of tasks than it would to do the work, maybe you are getting to detailed. But at times the little detailed tasks might be better in order to ensure nothing is missed.  Task at the level that is necessary.  If a single person would be doing all of the work you mentioned, does it really help to have each activity broken as a task. One of the biggest benefits of tasking, in my opinion, is to allow the entire Development Team understand the work and determine how/if multiple team members can participate in the work in order to complete the work more efficiently. 

Issue 2:team member as a developer has a responsible to run test cases(unit test and simple sanity doing by developer)?

Would anyone on the team intentionally do something that would cause problems or compromise the quality of the product? Who on the team has the skills to be doing the testing?  What is the best way to have the testing carried out and to have it available for future iterations? If you have others do code reviews, including automated tests would/should be included in the code review. This gives a second set of eyes to inspect for adequate coverage.