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

.

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?

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.