Darnassus wrote:
What do I do? Just plug it in and it'll show up? :f

Plug it in and use disk manager to initialize and format giving it a driver letter or choose you own

If you have a Optical drive I would change its drive letter to some thing other than D, E or F I usually use M or N for optical

you 1 TB spinner is probable using D So you might wan to change its letter to E or F, that should leave D open for windows to name your new SSD D

The only Issue with M.2 NVME is the Original OS, which coming from Asus factor is in RAID mode so that once the OS is install The raid driver is installed so your 2nd SSD will all ready have a driver loaded.

Then If you actually did the Clean install you probable changed The SATA Setting to AHCI and then the Native windows NVME driver will all ready be loaded. Either way you should have a driver already running for it.

Darnassus wrote:
Hold on I remember someone saying there's a max to how much these Laptops can address..

Is it they can only take up to 512GB total? Or they can take 2x512GB sticks?

Last I looked, the largest available are 2 TB SSD NVME M.2 you have 2 slots so you could have a total of 4TB if you desire. We are now dealing with UEFI with no limit to total storage other than your wallet. We are no longer dealing with MBR Which did limit us to 2 TB for a OS Drive.
We are no longer daling with he SATA controller with M.2 PCIe Bus With the only limitation is maximum speed for which we are approaching with the latest SSD running at 3000 Gbs Here is a better explanation of PCIe