I mean under which license is the source code available? I'm not sure if Unity is free software
Unity is available under the GNU Lesser General Public License version 3.
It is certainly protected by copyright. All creative works are - including those released under a free software or 'copyleft' license.
As far as I am aware Canonical hold no patents of any kind - of course it is possible other companies may claim it infringes on their patents.
There has been concern though with the Canonical Contributor Agreement - I've included a brief description below:
In the agreement, the contributor assigns copyright to Canonical, and Canonical simultaneously grants the contributor a very broad license back, so that the contributor retains full rights to re-use, distribute, and continue modifying the contributed code.
Some members of the free software community are concerned that in the future Canonical (or a company that takes over Canonical) could re-license Unity under a non-free license. Others see this as highly unlikely.
Quoting the upstream page at Launchpad: