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:

- quantum Hall effect, which has discrete resistances of type:
for integer values of $ν$.$R_{xy}=I_{channel}V_{Hall} =e_{2}νh $
- Josephson effect, which provides the Josephson constant which equals:
$K_{J}=h2e $