What is "~/Fotos Library.photoslibrary/resources/media/master" used for? Whenever I want to backup photos from a Photos Library I rely on copying all the contents of the folder ~/Fotos Library.photoslibrary/Masters.

However, I noticed that a lot of seemingly full-sized photos are stored in
~/Fotos Library.photoslibrary/resources/media/master with names as fullsizeoutput_XXX.jpeg.

What is this folder used for? Is it safe to get rid of those photos?
Photos image files are stored in the following location in Mac OS:

~/Pictures/Photos Library.photoslibrary/Masters/

The ~ tilde represents the users home directory, if you’re going to use the Go To Folder command to access that directory, do not skip the ~ prefix.

The Photos Library package file in Mac OS X

This focuses on local image files and not anything stored within iCloud. These are instead accessed through iCloud Photo Library or the Photos app.

How to Access Where Photos Files Are Stored in Mac OS

If you are going to manually navigate to the Photos file location using the Finder you would use the following steps;

Open the Finder in Mac OS and go to your home directory
Go to the “Pictures” folder
Locate the file named “Photos Library.photoslibrary”
Right-click (or Control+Click) on “Photos Library.photoslibrary” and choose “Show Package Contents”
Where Photos are stored on Mac

Open the folder named “Masters” to find the image files contained within Photos app
Photos file locations in Mac OS
The Masters folder contains directories of photos organized by date in year, month, and day subfolders. Within each folder are the image files from that specific date.

Note if you delete a picture from the masters directory it will no longer be accessible in Photos app. This is not recommended for obvious reasons. Copy a file from the directory if you wish to edit it.

This package folder is not intended to be user-facing which is why it is hidden, nonetheless you can access your files directly if you so desire.