They just apply their weird conversions on top of the metric system, but fundamentally they use the definitions of the International System of Units.
E.g. Pound is legally exactly 0.45359237 kilograms where kilogram is defined by setting the Planck constant h exactly to 6.62607015×10^−34 J⋅s (J = kg⋅m2⋅s^−2), given the definitions of the metre and the second.
Previously kilogram was a block of metal stored in France. But the copies of kilogram were diverging and there was no way of knowing where the changes were coming from. So now the kilogram is defined with regard to the physical properties of the universe as opposed to specific object.
E.g. a meter is exactly the distance traveled by light in a vacuum in 1/299792458 seconds. Where second is exactly the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.