Short answers, pulled from the story.
The International System of Units is the modern form of the metric system and the world's most widely used system of measurement. The abbreviation SI comes from its official French name, Systeme international d'unites. It is coordinated by the International Bureau of Weights and Measures, abbreviated BIPM.
The seven SI base units are the second for time, the metre for length, the kilogram for mass, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity. Their symbols are s, m, kg, A, K, mol, and cd.
The seven SI defining constants are the speed of light in vacuum, the hyperfine transition frequency of caesium, the Planck constant, the elementary charge, the Boltzmann constant, the Avogadro constant, and the luminous efficacy. Each consists of an exact numerical value paired with units, such as the speed of light fixed at 299792458 metres per second and the caesium frequency at 9192631770.
The SI was adopted in 1960 by the 11th General Conference on Weights and Measures, following an initiative that began with a study commissioned by the 9th CGPM in 1948. It is based on the metre-kilogram-second system combined with ideas from the centimetre-gram-second system.
The 2019 redefinition retired the International Prototype of the Kilogram and redefined the kilogram, ampere, kelvin, and mole by fixing exact numerical values for constants of nature including the Planck constant, elementary charge, Boltzmann constant, and Avogadro constant. The new definitions were adopted at the 26th CGPM on the 16th of November 2018 and came into effect on the 20th of May 2019.
The SI provides twenty-four metric prefixes that signify decimal powers ranging from ten to the minus thirtieth to ten to the thirtieth, with the most recent adopted in 2022. Most correspond to integer powers of 1000, and the only exceptions are those for ten, one tenth, a hundred, and one hundredth.
The SI is regulated by three international organisations established in 1875 under the Metre Convention: the General Conference on Weights and Measures, the International Committee for Weights and Measures, and the International Bureau of Weights and Measures. Decisions about units are collected in the SI Brochure, published in French and English by the BIPM.