To download torrents to Steam Deck, first you need to download the qbittorrent from the Discover store.
You can learn how to download apps using Discover Store here ⬇️⬇️⬇️
After you download the qbittorrent from the Discover Store, you can just open it with qbittorrent. From now on, you can just download the torrents using this program.
Quick tip: If you are using the internal drive and the game is too big, you can just select a folder from the MicroSD card, and the qbittorrent will download it there.
➡️ To do this
1) Open qbittorrent
2) Click on the preferences button
3) From the preferences menu, select downloads.
4) Scroll down and find Default Save Path and change the folder by pressing the folder icon and selecting a folder on the MicroSD card.
Now your torrents will download to your MicroSD card.