n00b question, sorry. If I had a desktop that could hold 4 HD and 2 SSD, could I turn it into a NAS? Could someone point me in the right direction if this makes sense?
You totally can, but since it will be on all day with 4 hdd look into wattages you want to live with. There are some small NUCs or Pi based NAS with low wattages. There is OpenMediaVault, FreeNAS/TrueNAS software to install
Note that there is some reliability drawback of spinning hard disks on and off repeatedly. maybe unintuitively HDDs that spin constantly can live much longer than those that spend 90% of their time spun down.
This might not be relevant if you use only SSDs, and might never affect you, but it should be mentioned.
You could totally turn on as needed, WakeOnLan is good for that. But typically when people run a NAS it is for streaming audio, video, file sync and backups and maybe docker running other services so the NAS is typically on 24/7 so it is available on demand. But it doean't have to be 100% uptime if you don't want it to be.
For example I have two OpenMediaVaults one on a pi and one an old IomegaNAS. The pi is on always with an attached drive, and serves Samba Shares and DLNA/DAAP shares. Has docker running syncthing, CUPS print server, Trillium Notes, and homeassistant; so makes sense for it to be on all day, especially because my wife's system backsup to it daily automatically. The converted Iomega NAS is mainly a backup machine sInce it is old and not as performant (only has 100 network speed. So that gets turned on to do a bulk backup and not much else.