The 8RC will have either a HDD (MSI seem to favor Toshiba drives - they are poor performers), or 2.5 SATA SSD. There is often an M.2 slot for either a sata or NVMe drive.
Note though that laptops are notoriously bad at cooling, and the 8RC IIRC has no screws on the base. There is one screw that has a "Warranty void if removed} sticker over it.
MSI expect you to get registered service centers to do the upgrades, and they are often slow and expensive. This is not limited to MSI though.
Because of the cooling issues with laptops, NVMe drives tend to run very hot. I currently have an Acer gaming laptop and every time my son plays fortnite on it the SSD throttles. This may not happen wth the GL63-8RC, but something to be aware of.
SATA SSDs don't run as hot, but you if you ever used a NVMe SSD on a laptop and then dropped to a SATA drive you'd probably be disappointed with the performance. I installed a Samsung 960 Evo 250 in my laptop (Acer are more accessible), and it boots up in a couple of seconds. On the HDD it took several minutes.
You might find these links useful...
https://www.laptopmain.com/laptop-m-2-ngff-ssd-compatibility-list/
https://au.msi.com/Laptop/GL63-8RC/Specification
The MSI site suggests the laptop comes with a SATA SSD, rather than a mechanical HDD.