With more context, I'll do my best to put together a helpful review of Sirina.
Streaming (downloading a temporary cache file) occupies a legal gray area in many jurisdictions, but hosting copyrighted content without a license is illegal. If Sirina Free Video is the official channel of a creator distributing their own work for free (via Creative Commons or similar licenses), it is 100% legal. However, if a third-party site re-uploads Sirina’s content without permission to make ad revenue, that is copyright infringement.
Высшая математика – просто и доступно!