The whole world uses both for various things. Even the countries that "officially" use metric. Specific global industries still use imperial. Canadian and British people are perhaps the most famous for combining the two, but most of Europe also mixes things in here and there.
And of course the whole conversation is Euro-centric and ignores the historical use of traditional measurement systems in Africa and Asia, but somehow that never gets brought up.
They are indeed usually in inches but that’s probably bleeding back to Europe from the US. And most people don’t actually know how much that would be in metric. It’s sometimes listed but no one I know actually uses those numbers. We just know that 65 is bigger than 55, etc. If we want to know if it fits in our living rooms, then we look at the actual size in cm. I also couldn’t think of anything else that’s imperial, at least here in Germany.
Rims are also commonly referred to in inches, at least in germany. But just as with screen sizes, it's 50/50 with cm, the latter being more useful especially with screens.
Yes, the measurement system is the same everywhere and you need the wheel size somewhere in there, but the size of the tire (if you were to take a slice of its profile) is in metric.
No, like I said, tires are sized in both. The width of the bead is given in millimeters, the diameter of the bead is given in inches, and the height of the sidewall is given as a percentage of the width aka Aspect Ratio.
Cut it and you're left with a U shaped slice measured in mm (even if it's a ratio that's used for both legs, if you apply a ratio to a measure in mm, the result is something you measure in mm). The tire itself is measured in mm and then the last number is the size of the wheel it's wrapped around.
That's the point of view I'm thinking about, the red part is the tire and you only need the first two (metric) numbers to know what it looks like, the wheel size doesn't influence the width or sidewall height of the rubber.
Also, past a certain point you're back to imperial measures (35/13.5 x 20 > height x width x wheel) or you might not even have the height/sidewall at all (14.9-42 is 14.9 width for 42" wheels but that's mostly for tractors.