Binaries for Windows and Mac are available at qbittorrent.org/download.php. There's no Flatpak or Appimage or Snap binaries for Linux, just archives with source-code. Linux users who feel like building it from source should note that qBittorrent requires libtorrent versions above 1.1.0 as of 4.2.0 and libtorrent 1.2.3, released on December 19th, is absolutely required if you plan on using it with a SOCKS proxy (it has a buffer overflow fix for the SOCKS5 UDP logic). The Windows and Mac binaries available at qbittorrent.org/download.php are built against libtorrent 1.2.3.