Wait so let's say I'd like to use my existing SSD boot drive from my old Mac Pro, it sounds like that it wouldn't be possible to just pop it in and go? I'd have to use the boot drive that's shipped with the nMP and then copy over my data?
Depends upon just how old the Mac Pro is. If the Mac Pro doesn't run macOS 10.15 then you don't have an OS that is compatible with the new Mac Pro. ( If running "Bubba's hack to make it fit" then that is a 'don't have' status. So "old" as in MP 2013 there is a path. "old" as in 2009-2012 no.
If you boost to latest, greatest macOS that is new than what the Mac Pro 2019 launches with then can get into the "insert a 2nd drive space". You'd need to boot off the internal drive ( and pragmatically leave a small nominal macOS instance there). Create an account. Then can go into recovery mode and lower the threshold on secure boot ( boot internal T2 drive only). . Then simply need a mounting bracket/card for your SSD.
As another poster pointed out there is a path using Migration assistant. You can migrate over Users and possible apps to the new system. ( If this disk's OS has been upgraded over 5-6 times. it may be prudent to do a clean app upgrade at this point. Reinstall the non MacApp store apps with modern installers. More than likely have kernel extension cruft that are betting that Migration assistant is going to automagically filter out of you. ).
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.... Ok, but what about the slots where Apple is gonna put their SSDs on to?
It has two slots, so let's say I get the 2x512gb option, and later would upgrade to 2x1TB. Do you think it will be possible to swap those SSDs out somehow? Or are they gonna be soldered or something?
Those aren't SSDs. What Apple has done is implement a single SSD with three (sometimes 2 ) physical parts. The T2 contains the SSD controller (the brians). Those slots that the cards fit into a just storage NAND chips. Not a SSD. There is a small communication buffer between the chips on the card that is used to communicate back the SSD controller in the T2; but the cards are super duper 'dumb'.
To later "upgrade' is essentially getting new
internal components of a SSD. Pragmatically it is more like buying a new one than a upgrade. The cards will have to be paired to the T2 (and SSD controller). That should effectively wipe out and reset the whole SSD. If there is no backup the data is gone. The meta data the T2 uses about the wear conditions on the drive and much of the other maintenance control will wiped out with the change. ( pragmatically these should only be new , unused, NAND daugther cards ). All the metadata gets toasted with the regular data is yank the old cards.
There are no 3rd parties who "upgrade" other people's SSD internals. So there shouldn't be much expectation that someone else is going to upgrade the internals of Apple's SSD either.
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I’m not sure where ibuildmacs.com is getting there technical information about the 2019 Mac Pro but they show Slot 1 and Slot 2 inhabited by NVMe SSDs that use the PCIe 3.0 controller, encrypted by the T2 chip.
That basically disagrees with the Apple block diagram for the iMac Pro. Folks shouldn't get all twisted here. This drive implementation foundation for the Mac Pro has been in deployment for two years at this point. It extremely likely is the same basic system that has been running in the iMac Pro.
The cards may have a diferent color scheme than the iMac Pro cards, but is the same T2 so likely the exact same methodology. The T2 doesn't 'talk' NVMe to the card. There is a special subset of PCI-e that Apple uses ( sPCI-e or something like that. )
