'Photos' and file storage locations

See the following from Overview of importing photos and videos into Photos on Mac - Apple Support

By default, imported photos are copied into the Photos library. If you prefer, you can store photos and videos outside the Photos library—for example, in a folder on your Mac or on an external hard drive—and still view them in Photos. If you ever need to move content stored outside the library into your Photos library, you can do that, too. See Change where Photos stores your files.

In other words, by default they are duplicated. By default, imported photos are stored in Home > Pictures > PhotosLibrary.

You don't access or manipulated the content of your Photos Library directly -- you use the Photos app to do that.

TOMUNOID wrote:

1 - If I import these photos into the Photos app, what happens to the storage location?

2 - Does the storage location stay the same, but the photos just appear in the app?

3 - Does it duplicate the files and store them in a new location when they are imported? Is the location moved?

I don't want to end up with having the same photos stored across multiple locations, but also don't want to delete anything by accident.

There are two types of libraries, managed and referenced. Managed is when you copy the image files into the library and let Photos manage them. Referenced is when you don't copy the file into the library but reference them from the Photos library. Before you jump on the reference library be warned that Photos doesn't handle reference libraries as well as Aperature did and there is a danger of losing the links easily. See this user tip and consider it seriously: Disadvantages of a Referenced Library in Photos for Mac

1 - as I mentioned previously with the recommended managed library the image files are copied into the library to the Masters folder.

2 - no. The image files are copied into the library .

3 - yes but after confirming that the photos are safely imported your can delete the source folders you currently have.

With Photos 4.0 you can import a folder containing other folders of image files and have each folder of image files end ups as an album with the same name as the folder had.

OR you can use this AppleScript to select and import your folders of image file into the library and have the album titled the same as the folders (this will eliminate the many folders that get created by the method in the animated screenshot about): Import Folders of Image Files into Library as Albums. The script compiled as an application can be downloaded from this tutorial site:  P01 - Applescripts from Photos’ User Tips Compiled as Applications

I would also recommend that you review the entire document: Overview of importing photos and videos into Photos on Mac - Apple Support

and that you don't rush to delete anything until you are sure you are happy with the way things work with Photos and imported photos.

If you are using the default option described in my previous reply, deleting the original won't delete the imported photo.

The setting is at Photos > Preferences > General > Importing > Copy items to the Photos Library.

If you have any doubt or are unsure, take a photo that you don't want to keep and test it.