Hi Mathew,
the predefined physical units are visible (and can be maintained) in the Business Configuration activity "Quantities and Units of Measure“. There you find also the conversions to reference units of the same type (e.g. all lengths referenced to meter, all weights referenced to kg).
(In the product master, you would define conversions between either Trading units (e.g. how many each are in a box) or between units of *different* quantity types that are valid for this material - but not between physical units of the same type.)
Regards
Michael