You can add media files to your GarageBand project by accessing them through the media window and dragging them into your timeline.
Media Button
Click on the Media button, which is the far right button in the bottom right corner. This will open up a right sidebar section to access media files.
Browse the media on your computer by using the media window. You can choose audio files, photos or movies to add to your GarageBand project.
Click and Drag the Media File
When you've chosen a media file, click and drag it onto your timeline. It will now be part of your project.
