They were all done by scientists or engineers.
The meter was defined based on what they calculated as 1 millionth of the length of Paris’ meridian.
The second was 1/86400 of a day, which makes sense with the angle/circle nomenclature on the clock.
The gram was initially set to be the mass of 1cm³ of water at 4°C - which is why 1l of water ≈ 1kg.
Of the SI system. The metric system started with cgs (centimeter, gram, second) and evolved from there.