There are tons of computers running Linux besides PCs. By far the biggest part are servers and supercomputers. Microsoft even has their own distribution for their server business. Then there are all the Android phones and devices. Android is Linux. In Germany I've also often seen Linux used for kiosks at government agencies.
Linux is used in TVs and set top boxes. Everything that says Tizen or WebOS is powered by Linux. I've also seen it used as in-flight entertainment systems. And Lunduke had an example of Linux running on a machine controlling how cows are milked, if I recall correctly.
For most systems you won't actually know what OS is used until you see a hardware error screen. Although Microsoft has made it a little easier with mandatory updates.