In a default Qlik Sense Enterprise installation there is a Stream setup called "Everyone", which all users should have access too.
There is also a space called Personal, under which there is a stream called Work, which only you can see.
There is no default stream called "My Stream" though this may be a custom setup of your environment. Qlik Sense Administrators can setup whatever streams are required, and configure these with any security rules they desire. If you are seeing a stream called My Stream I would contact your administrator to confirm who can see it.