Mac Pro 2019 Hard Drive capacity and other questions

The Finder and MacOS in general only deal with Volumes: sections carved out of physical drives and 'dressed up' to hold files.

Based on your screenshots, you may indeed have a 4 or 5 TB boot drive in your machine.

launch Terminal and type this command to see the underlying structure.

Post the text, an screenshot or a photograph of the output.

diskutil list

.

From that view, it looks like all the 10.15 and 12.6 stuff is on one 2 TB drive, and a separate 2TB drive is named 'Hard Drive 2TB'

Could you possibly have Physical TWO 2 TB drives inside, plus the 8TB?

Please run Disk Utility, use the little menu to "Show all Devices", and post its top-level view that way.

re-allocating space, adding or removing partitions, has alway been scary. One false move and your entire drive contents could be gone in a moment.

¿what is the date of your most recent complete and Trusted backup, and by what method?