Original vs Master

The messy Originals folder with the renamed originals is the replacement of the "Masters" folder, introduced by Photos 5 on Mojave a bad surprise for the users. Currently there is no longer a way to force Photos to keep the "Originals" folder readable for the users. The new filenames are unique hexadecimal codes. They are the same across my Catalina macs, perhaps they help with the iCloud syncing.

If you follow the device given by TD and export all Photos with "File > Export > Export unmodified Originals", Photos will restore the original filenames for the exported files. And you can select a subfolder format "Moment Name". Then the exported photos will be saved in subfolders named by the location and the date. As an added bonus you could enable the checkmark "Export IPTC as XPM", then Photos will save the titles, keywords, descriptions, locations in a sidecar file. And if you import the photos back to photos with the XPM file present, the IPTC metadata will be restored from the file.

You have upgraded your Mac from old version to Mac OS Catalina 10.15.2 , in the previous version iPhotos were used , but once upgraded the photo library is also upgraded see the article https://support.apple.com/en-in/guide/mac-help/mchle1529919/mac

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 Original 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

Note : you must not drag and drop separate subfolders from originals folder to the external hard drive it can corrupt your photo library .

Second method is https://support.apple.com/en-in/guide/photos/pht40b0be510/3.0/mac/10.13

And export them https://support.apple.com/en-in/guide/photos/pht6e157c5f/mac

to a desired location say download folder in finder and connect an external hard drive to the Mac , then transfer the photos from download folders into the external hard drive , hence all your desired photo data is saved .

If you want to back up the originals, then use the File -> Export -> Export Unmodified Originals command.

Nope, messing inside the library package is bad juju. Why not back up the whole library?

You could upload them to iCloud, delete the old library and create a new one from iCloud.