Olympic swimming pools are usually defined in metric units for length, i.e. 50m long. My guess is that their width and depth are also specified/defined in metres. The contents therefore being in thousands of litres or cubic metres, though of course may well be expressed in gallons which requires some sort of conversion from one system of measurement to another.
However, the use of 'Olympic swimming pool' or 'football pitch' or 'double-decker bus' etc. as metrics for describing the volume or area of something else, doesn't require them to be defined in metric units per ce. In the same way that something defined as being small enough to fit on the head of a pin. We instinctively know that a swimming pool or football pitch is large and that the head of a pin is small (but not microscopically small). So these terms are useful for establishing a perception of scale rather than an exact measurement of size. I don't even know what the typical size of the head of a pin is, but it's usually pretty small though not as small as the point.
We in the UK, adopted the SI (metric) system of measurement decades ago. I was at school and college at the time, had to learn both and had to take exams that could be in either. In just over forty years of engineering, most of the projects I was involved with required me to work in metric units. A big difference was when working with the US, when I ended up having to work in metric, imperial and whatever it is the US use which seems to be a strange mixture of everything.
Socially, I've always gone down the pub for a pint, think of my weight in stones (even though I know it better in kg), my height in feet and inches (I only roughly know what that is in metres), my waist and chest sizes are inches, I have no idea without thinking about it, what they are in centimetres and I take size 9 shoes which I think is 43 in European sizes and I have no clue how that relates to the metric measurement system. Overall, it's a bit of mess but somehow it sort of works most of the time.
And if someone wants to put an AMG badge on their non-AMG car and whether that is in inches, millimetres, chains, furlongs or light-years is all fine with me.
