What are the real world measurements in Unity?

I know the Terrain is measured in metres, so a Terrain object with a width and length of 1000 is 1Km^2.

Does that mean everything in terms of distance is measured in metres? I believe that magnitude is measured in m/s, so that would lead me to believe it follows the pattern of metres and seconds for measurements.

Can someone just confirm this please?

Yes. Everything (especially physics) is in meters.

Having said that, if you aren’t using Physics, then the Units are arbitrary, and you could scale-up and down, or define them as your own units.