Short answers, pulled from the story.
The International System of Units anchors its entire structure on seven specific base units. These units define fundamental physical quantities like time, length, and mass through the second, metre, kilogram, ampere, kelvin, mole, and candela.
The 11th General Conference on Weights and Measures adopted the International System of Units in 1960 as the modern form of the metric system. This adoption followed a 1948 study commissioned by the 9th CGPM that assessed global measurement needs for science and education.
Scientists proposed fixing numerical values for seven constants of nature instead of relying on physical objects to ensure stability across scales from atomic particles to astrophysical measurements. The new definitions were adopted at the 26th General Conference on Weights and Measures on the 16th of November 2018 and came into effect globally on the 20th of May 2019.
Three international organizations manage SI standards under the terms of the Metre Convention established in 1875. These bodies are the General Conference on Weights and Measures, the International Committee for Weights and Measures, and the International Bureau of Weights and Measures.
The second measures time through the hyperfine transition frequency of caesium-133 atoms. A single second equals exactly 9,192,631,770 periods of radiation from this atomic transition.