Backup Library to External Drive

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

Read these articles also https://support.apple.com/en-in/guide/photos/pht6d60d10f/mac

https://support.apple.com/en-in/guide/photos/pht6d60348f/mac

To add to what Terence said the original files inside the Photos library do not have the file name that the camera/iPhone gave to them. That are changed to some "weird" combination of letters and numbers to facilitate better syncing with iCloud Photo Library.

So if you want to have any useable file name for your original image files export them out of the library as follows:

Use either the File name (it will use the real original file name) or the Title if you've retitled them. Also if you want to include any Titles, Descriptions or Keywords then check the Export IPTC as XMP check box.

A) Use an external drive to store all originals for my main photo library

However, since you already have A what you're proposing is rather redundant.

What is the best way to do this? Open package contents, drag over the Originals folder to a hard drive

Under no circumstances. Never access the elements of the Photo library via the Finder. It's just not supported in any context and risks significant data loss.

I'm not sure exactly what you want to do. When you say this:

I'd like to have a separate volume to keep and maintain library ORIGINAL files (the files within the library package). 

Do you mean a: you want to back up the original photos? or b: you want to store your originals external to the Library package (which I strongly advise you do not do)?

If you want to back up only the original files then export them

File -> Export -> Unmodified Originals.

You can access the recent imports via the Import heading in the Library.