A very raw guide with an example of how you can use Qbitorrent on any computer as NAS:
Connect a computer, tablet or server that you dont use much to your local home network (router or switch).
Install Qbitorrent on it.
Connect a USB hub to it and connect a couple of HDD's.
Enable Qbitorrent web interface in settings.
Check the local IP the server/tablet/computer has (This can be done in Windows by clicking Start, writing CMD to open a command line interface and writing "ipconfig" you will see a number like 192.168.0.123 or similar, or you can check in your router for connected machines and see what local IP it has. Google for more info.)
Go to your normal computer and make sure you are connected to the same router/network.
Connect to the 192.168.x.xxx:8080 IP and port (change the x's for your IP) in your internet browser and use the web interface to add and remove torrents.
Share the HDD's on the server/tablet/computer with the USB hub on the local network.
Connect to the HDD's via the local network and watch the torrents downloaded anywhere at home from any device using for example VLC player.
You can also "map the network drives" on Windows so they become part of "This PC" and you see them every time you open Windows explorer.
If you want to, you can also install a remote desktop program like Rustdesk or TightVNC or even use Windows remote desktop feature, or any other solution, or use Putty if its a Linux server, to connect to the NAS server remotely to make updates or changes. That way you can have it in a wardrobe or under the bed or in the garage etc without having to physically go there to make any changes.
Note: You might need to set a static internal IP on the NAS or it might change when the router restarts. This can be done both on the NAS or in some cases in the router by binding it to the MAC address. Alternatively check the new internal IP if it changes, and change so you connect to the new one instead. It's relatively easy.
You can also create qbittorrent "Watchfolders" on one of the shared HDD's, one for each category, so you can add .torrent files from other computers just by dropping them in the right folder, its really handy, don't have to open the webgui, just drop the torrent in a folder.
Pros: With a NAS you can seed 24/7 but still turn off your normal computer at night. You can connect with any device and have access to the downloads. You can have all discs and cables out of sight.
Cons: You need to fiddle with the installation and make tests until it works. You need to remember to dust off and clean the area with the NAS often. But its well worth it!