whenever the admin creates a issue, it is been seen by all the users. how to restrict so that only their assigned tickets can be seen by them
To achieve it, i think you can use docshare.
Here some reference:
https://docs.erpnext.com/docs/user/manual/en/sharing
And if you want to make it automatically used server script, and hooks to it.
Hope this help 
You can refer to this