TODO how does basing it on the elementary charge help at all? Can we count individual electrons going through a wire? 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:
$R_{x y} = \frac{V _{Hall}}{I _{channel}} = \frac{h}{e ^{2} ν}$
(1)
for integer values of $ν$ .
Josephson effect, which provides the Josephson constant which equals:
$K_{J} = \frac{2 e}{h}$
(2)