How do I back up my entire photo library

Photos can’t be transferred directly from iCloud to an external hard drive for that you need to set up iCloud photo library see the article https://support.apple.com/en-in/HT204264

And its preferences https://support.apple.com/en-in/guide/photos/pht5156cc968/4.0/mac/10.14 , open finder > Go > home > your user name > pictures either drag and drop Photos Library.photoslibrary to an external hard drive , the second method is right click on Photos Library.photoslibrary and click on show package contents in the upcoming column , you can also drag and drop complete masters folder to an external hard drive that is connected .

Also as per the article https://support.apple.com/en-in/guide/photos/pht6d60d10f/mac

• Manually copy your library to an external storage device: Drag the Photos library (by default in the Pictures folder on your Mac) to your storage device to create a copy.
• See this article also Photos library - Move your Library to an external storage device