Even if you don’t want to use it as a computer having a Bluetooth keyboard is almost a must-have because otherwise configuring non-steam games in windows mode is a real pain.
Once you enabled SSH on the Deck, you can access the filesystem using SSHFS. If your desktop is windows, you can install this program: https://github.com/winfsp/sshfs-win/releases/tag/v3.5.20357. In Linux (Debian derivatives like Ubuntu) install it wit sudo apt install sshfs then read the man páge with man sshfs to learn how to use it.
If you're a little less CLI savvy, KDE Connect will be much easier. Winpinator on your desktop and Warpinator on the Steamdeck works well and can sometimes help if file copies via KDE Connect fail.
SSH is weirdly slow however. There is a command to disable wifi powersaving (or something like that) which is the cause but I couldn't be bothered lol.