It works by starting a web server, making it accessible as a Tor onion service, and generating an unguessable URL to access and download the files. It doesn’t require setting up a server on the internet somewhere or using a third party filesharing service. You host the file on your own computer and use a Tor onion service to make it temporarily accessible over the internet. The other user just needs to use Tor Browser to download the file from you.
To learn how OnionShare works, what its security properties are, and how to use it, check out the wiki.
You can download OnionShare for Windows and macOS from https://onionshare.org/. It should be available in your package manager for Linux, and it’s included by default in Tails.
You can set up your development environment to build OnionShare yourself by following these instructions.
