The base 2019 Mac Pro only has a 256GB SSD; pathetic for a $6k computer.  However, the bottom of the specs page on Apple’s site mentions Pegasus MPX Modules.  The Pegasus 2Ji gives you a couple of 3.5” drive slots, and the 4Ri will give you an internal 4 disk RAID 5!

https://www.promise.com/us/Promotion/PegasusStorage