The film industry is mainly Windows and Linux, the larger the facility the higher the chance of it being a Linux house, but not always.
You hear "Macs make movies" so often and if anyone who doesn't work in the industry could see the state of things they would find the quote funny.
The main users of Apple products are producers and other pencil pushers running around with iPhones and Mac Pros, video files passed around during production are usually QuickTime files. Other than that a very limited amount of the business is anything Apple related. Final Cut is popular however its only one tool in the whole production chain and a facility doesn't need to equip everyone with a Mac just because its used. It is not uncommon to see a few Macs around facilities however you do not seem them in numbers.
I have only visited one all Mac based facility and even then they had RHEL dual booting macs. I can probably count on both hands how many visual effects studios I've seen or heard of that are all Mac based. Linux and Windows are no doubt the prominent players in the visual effects field. Linux isn't just on the backend either, plenty of facilities run Linux on the desktop and any one day there are thousands of artists typing grep and ls in a terminal somewhere. Its not just the bigger facilities either, plenty of smaller shops run Linux all around. If a facility isn't running Linux it most likely is running Windows.
Avids are used in editorial nothing more, so you do not see them in numbers. The Autodesk offerings have moved off Irix and are now HP/IBM Linux workstations.
Nvidia has the DCC market cornered with the Quadro line, I'm not going to get into the debate of whether they are wroth the price or not though. Even if a facility doesn't use Quadros in mass you can almost be certain the workstations have some sort of Nvidia card installed. Most applications in DCC are OpenGL based, since Nvidia has a track record of having a better OpenGL product, its not hard to understand why they might have a strong hold on the DCC industry.