Comment by minhmeoke

If you don't have a network connection, you can setup an adhoc hotspot on any Android 9+ (Settings > Network & internet > Hotspot & tethering https://support.google.com/android/answer/9059108) or iPhone (Settings > Personal Hotspot), then connect to it using any WiFi-enabled device.

A useful filesharing utility that a friend wrote is https://github.com/akovacs/uploadserver - it's basically a nicer version of:

    python -m http.server 8000

Download prebuilt binaries for Linux, Windows, Mac OS from https://github.com/akovacs/uploadserver/releases/ or install from source if you prefer

Determine your machine's IP address using:

    # Linux
    hostname -I

    # Mac
    ifconfig

    # Windows
    ipconfig

Start the file server, and then navigate to it using the web browser of your choice on any device connected to the WiFi network (no need for a client application).

    chmod +x upload_server
    ./upload_server

Navigate to the server's ip address (indicated by the hostname -I command you ran earlier) port 8000 in the browser of your choice and upload files using the web UI or directly via curl:

    curl -X POST --data-binary @file_to_upload.txt http://192.168.1.X:8000/uploaded_file.txt

Then download the file to another machine or mobile device either from your web browser or via a commandline tool:

    curl http://192.168.1.X:8000/uploads/uploaded_file.txt --output downloaded_file.txt