It’s easy, just type is:unread and press search. This will search for EVERY email that is unread.
Search For Unread Labeled Email
I suggest that you setup filters and manage your incoming emails categorized by labels. For example, this will allow you to search for unread email labeled Business.
Search For Unread Nested Labeled Email
As for nested labels, you simply need to add a “-” between the parent and child of the label relationship hierarchy, for example
Search For Unread Email in Inbox ONLY
I’m glad you asked. The inbox is by itself a label too!
So you would simply use:
Hope that helps.