At the introductory level it's indeed very hard if not impossible to introduce students to the new SI. The SI's purpose is not to provide didactically feasible and simple definitions of the units but to provide as accurate standards as possible given the contemporary technology of metrology.



To achieve this accurateness, however, in as a technology-independent way as possible, one uses what's to the best of our knowledge of today are fundamental constants to define system of units. These constants are Plancks constant ("action quantum") ##h## and the speed of light in vacuo, ##c##, and the charge of an electron, ##-e##.



Now one needs one more constant to build up the system of units. The natural choice would be the Newtonian gravity constant ##G##, but that's the bete noire among the natural constants that cannot be accurately measured today. That's why there's still one material-dependent constant left, and that's ##\Delta

u_{\text{Cs}}##, i.e., the frequency of the groundstate hyperfine transition of Cs-133, defining the base unit second since 1967 by setting its value to 9 192 631 770 Hz, where Hz=1/s is the unit of frequency. Based on this everything else follows with the constants stated above: The speed of light is fixed to 299 792 458 m/s defining the base unit m based on the base unit of time, s. The kg then is defined via Planck's constant which since 2019 set to ##6.626 070 15 \cdot 10^{–34} \text{J} \cdot \text{s}## via the use of the already defined units m and s given that ##1 \text{J}=1 \text{kg} \cdot \text{m}^2/\text{s}^2##. Setting the elementary charge to ##1.602 176 634 \cdot 10^{–19} \text{C}## defines, again under reference to the above defined s, to the base unit Ampere for the electric current given that 1 C=1 As. For the temperature unit, K, one needs to fix another constant, the Boltzmann constant ##k_{\text{B}}=1.380 649 ⋅ 10^{-23} \text{J}/\text{K}##. Finally, now also the Avogadro number, defining the unit 1 mol of a substance as the number ##N_{\text{A}}=6,022 140 76 \cdot 10^{23}/\text{mol}##.