Let’s say the files you want to copy are in ~/Downloads.

Start a local HTTP server on your laptop:

$ cd ~/Downloads
$ python3 -m http.server
Serving HTTP on :: port 8000 (http://[::]:8000/) ...

Find the IP address of your laptop within your LAN:

$ ifconfig  # macOS
$ ip addr   # linux

Now go to your Steam Deck, access http://<ip>:8000 via the installed web browser, and download your files.

Alternatively, run wget / curl in a terminal.