Brave built-in torrent viewer use WebTorrent. To use it, basically you only need to find the magnet link. Click on the magnet link and it’ll take you to the download page. Clicking Start Torrent button at top right will start downloading the file.
In this case, if you only click Start Torrent, then it may like @Steerider questions about where the files be saved. But if you also click the save button, then it should ask you where to save the files.
 Example magnet link https://webtorrent.io/free-torrents
 Windows on
 Linux on
 I forgot the location on Mac. But it should be under Mac’s