By default, torrents are downloaded to the music or video library in the app. However, if you give the app permission, you can change the download location to any folder on your phone. To do this, tap on the hamburger menu icon, then tap on Settings, tap on the "change" option next to Default Download Folder, choose the folder of your preference then finally tap on OK. When you download new torrents, they will download to this new default location.
If you wish to change the download location for a single torrent already downloading, go to the torrent list view, then tap on the torrent to view its files, tap on the three dots menu (Kebab menu), this will reveal the overflow menu. From there, tap Move File. The directory will open and you will be able to choose any destination from there.