Deleting Photos From "Photos" App Does Not Delete Them From Local Drive
Photos app stores the pictures in a database. There will be an original copy of the image, a copy of the image if you edit it in any way as well as proxy / thumbnail photos so that various functions are fast.

These files are all stored and managed via the application and default are stored as a directory or folder on the filesystem that is shown as one icon. Look in your Pictures folder in your home folder. (Shift Command H in Finder to short cut to your specific home folder).

Any photos stored outside that location can be deleted using finder, search tools, etc.. and using Photos won't help manage them. Any photos stored inside the photo library should not be deleted, so they are generally hidden from search to prevent people from breaking the library database.

Most people don't ever have more than one Photos library, but here are documentation for those that prefer to know how things are stored.


https://support.apple.com/en-us/HT201517 Explains how to move your photos library - you could move yours to an external drive and then delete it. Then you'll know all the searchable photos can be deleted, saving your library to be moved back once you are done cleaning.
https://support.apple.com/en-us/HT204414 Explains the system library and how to select alternate libraries when you start up Photos app by holding the option key.