Hi scottyip,
Per your first question. As you are an administrator you will be able to see all attachments etc. from other users by default.
Also by default every other user is only able to view their own attachments etc.
So, if you as an administrator want to view only your attachments you will have to check the box “Enable so Admins can only view their own attachments” as described here https://ps.w.org/wp-users-media/assets/screenshot-1.png?rev=2293948
The settings on https://ps.w.org/wp-users-media/assets/screenshot-2.png?rev=2293948 are only attended to be checked if you have an user role that is supposed to only view their own attachments and not everyone elses. If you perhaps have 3 out of 8 user roles as an example that needs to view their own you attachments you check them here, the rest of the user roles will be able to view all attachments from all user roles.
You can use those settings for your liking and needs.
The plugin works perfectly as I tested it out now.