How does photos app in mac imports pictures from hard disk? I copied some pictures in my mac from my phone using a usb. But in photos app some of them are being shown and some not shown. Some of them I have deleted are being shown and some are not being shown after deletion from finder as jpeg files. My question is how does photos app work?
The Photos app works by creating a library of your photos files.   A regular library has two important parts, the books on the shelves and some kind of cataloging system to help you to find the book that you are looking for.  If you brought a cookbook that you bought into a real library, other library patrons wouldn't be looking at your cookbook because it is not a part of the library's collection.

In the same way, if you copy pictures onto the hard drive of your computer, but do not tell the Photos app about them, then they are not considered a part of the Photos library.

There are several ways to import photos into the Photos App.  Photo's usually opens when you connect a phone, for example and will attempt to import photos that way.  If you have a folder with pictures though,  you can either drag that into the window of the Photos app or from the Menu Bar with Photos open click File>Import.  For more check out https://support.apple.com/kb/PH21297?locale=en_US