A NAS is basically some software running on a computer, so you can use a desktop as that computer, ideally with a light operating system (for example, Linux in text only mode).

HOWEVER: desktops are designed for far higher computational loads than needed by a NAS, plus things like graphical user interfaces and direct connection of user peripherals such as mice, so even when idle they consume a lot more power than the kind of hardware used in a typical NAS.

Also the hardware in a good NAS will have things like extra higher speed connectors for HDDs/SDDs (such as SATA) rather than you having to use slower stuff like USB.

So keep in mind that a desktop as NAS will consume significantly more power than a dedicated NAS (as the latter will probably be running on something like an ARM and have a power source dimensioned for a couple of HDDs, not to run a dedicate graphics card like a desktop has) and probably won’t fit as many disks.

If you’re ok with having most disks be accessed a bit slower and USB3 work for you (and, for example, if your NAS is on 100 Mbit Ethernet, it’s the network that’s the slowest thing, not USB3) then it’s usually better to use an old notebook rather than desktop because notebooks were designed for running of batteries hence consume significantly less power.

Frankly I would advise against using an old desktop as NAS mainly because in a year or two of continued use you’ll have paid enough in extra electricity costs vs using a NAS to pay for a simple but decent dedicated NAS.