Can someone provide more clarity than is found at https://www.dell.com/support/article/us/en/04/sln312251/upgrading-storage-in-the-dell-precision-5820-7820-7920-tower-workstations?lang=en

There are a few videos, but there's no voiceover (just mindless instrumental music), so it's hard to know what is going on at every step (and why).

The closest I've come to is part number 400-AXJF, but that looks like it requires some other kit which Dell are telling me is no longer available.

I'm not sure what's happening inside Dell, but this documentation for a workstation-class PC is really sub-par.