The qBittorrent source code is licensed under the GNU General Public License, version 2 or (at your option) any later version (GPLv2+).
However, the binary distribution is licensed under GNU General Public License, version 3 or (at your option) any later version (GPLv3+),
because it contains GPLv3+ assets (eg images). In both cases, the following special exception is added:

In addition, as a special exception, the copyright holders give permission to
link this program with the OpenSSL project's "OpenSSL" library (or with
modified versions of it that use the same license as the "OpenSSL" library),
and distribute the linked executables. You must obey the GNU General Public
License in all respects for all of the code used other than "OpenSSL".  If you
modify file(s), you may extend this exception to your version of the file(s),
but you are not obligated to do so. If you do not wish to do so, delete this
exception statement from your version.

See also the AUTHORS file

See COPYING.GPLv2 file for the GPLv2
See COPYING.GPLv3 file for the GPLv3