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The key is to define only the minimum number of measures: if you define more definitions become over constrained and could be inconsistent.
Learning the modern SI is also a great way to learn some interesting Physics.
Video 1. Source.
Great overview of the earlier history of unit standardization.
Gives particular emphasis to the invention of gauge blocks.
TODO how does basing it on the elementary charge help at all? Can we count individual electrons going through a wire? https://www.nist.gov/si-redefinition/ampere/ampere-quantum-metrology-triangle by the NIST explains that is it basically due to the following two quantized solid-state physics phenomena/experiments that allows for extremely precise measurements of the elementary charge:
Unit of mass.
Defined in the 2019 redefinition of the SI base units via the Planck constant. This was possible due to the development of the kibble balance.
Measures weight from a voltage.
Named after radio pioneer Heinrich Hertz.
Mega-Hertz, i.e. a million Hertz.
Video 2. How a quartz watch works by Steve Mould (2017) Source. Mentions feedback loop loop with the quartz tuning fork for the piezoelectricity and an amplifier. Also mentions the choice of 32768 Hertz () as the first power of 2 that is outside of the human hearing range, and then how a frequency divider is used to reduce the frequency to get the second counter.
Video 3. How an atomic clock works, and its use in the global positioning system (GPS) by EngineerGuy (2012) Source. Shows how conceptually an atomic clock is based on a feedback loop of two hyperfine structure states of caesium atoms (non-radioactive caesium-133 as clarified by the Wikipedia page). Like a quartz clock, it also relies on the piezoelectricity of quartz, but unlike the quartz clock, the quartz is not shaped like a tuning fork, and has a much larger ressonating frequency of about 7 MHz. The feedback is completed by producing photons that resonate at the right frequency to excite the caesium.
Uses the frequency of the hyperfine structure of caesium-133 ground state () to define the second.
International System of Units definition os the second since 1967, because this is what atomic clocks use.
A series of systems usually derived from the International System of Units that are more convenient for certain applications.

Ancestors