Network Attached Storage [NAS] is a standalone, separate device housing one or more hard disks [or SSDs] that allows network connected devices to access the files stored on it. An NAS has a processor, RAM, & runs an [often custom] OS, providing restricted access & security. It's common for an NAS to use 2 or more mirrored hard disks, where all data is written to both disks, so data's preserved if one disk fails. While an NAS that only accepts one hard disk [not included] can start around $100 on Amazon, they more typically run from a few hundred to thousands of dollars.
wikipedia[.]org/wiki/Network-attached_storage
A Windows Storage Pool OTOH provides the same sort of storage but connected to a PC or laptop rather than your network -- that PC or laptop would have to be set up to share that storage if you want / need network access. It's cheaper to implement -- all you need are the hard disks & some way to connect them to a PC or laptop -- and odds are it's more reliable, since there's much less electronics & no separate OS involved. Writing to the disks is also usually faster, compared to data transfer over a network. A potential downside, maintenance of a storage pool is mostly done using the Command Line or PowerShell, while an NAS might provide an easier GUI in your web browser.
If you set up a Storage Pool to combine storage using however many disks [you just see one large disk in File Explorer] the disks can be any size -- if you set up mirroring it's best if the drives are identical, or at least the same size. Creating / setting up a Storage Pool is easy -- just follow the simple prompts in Settings -> Storage, or Control Panel -> Admin Tools -> Computer Management [Win10], or Windows Tools -> Computer Management [Win11].
A Storage Pool will work with disks connected via USB, but there's a catch -- in my experience you need to be running Win11. Once you set it up the Storage Pool's configuration is stored on the disks, so the Storage Pool is portable -- any copy of Win11 will see and access the Storage Pool once it's connected.
Background...
I've used an NAS for many years to store copies of backups, VMs, photos etc. [how I came to have one is a long story], but it's been showing its age, and in preparation for its eventual death I picked up a dual drive dock last year, planning on setting up a Storage Pool when the time came. Well, that time was yesterday, so I pulled the 2 hard disks out of the housing, stuck them in the dock, connected the dock to this PC, deleted the Linux partitions, and proceeded to *try* and set up the Storage Pool in Win10. I wound up trying with multiple copies of Win10... either the documentation lies, or it left out the part where it will only work with Win11, or the drive dock I used has some peculiarity that's incompatible with Storage Pools in Win10. I found complaints online, but no solutions. After too many hours wasted I tried it in Win11, where everything worked like it should, though once the Storage Pool was set up it would not work in Win10. I could not find compatibility info for the drive dock, or any drive dock for that matter, so I bit the bullet & started upgrading copies of Win10 to Win11. $25 for the drive dock vs. around $300 for a [hopefully] decent NAS was all the incentive I needed.