As per [Wikipedia](https://en.wikipedia.org/wiki/International_System_of_Units), the new definition of kilogram is, “The kilogram is defined by setting the [Planck constant](https://en.wikipedia.org/wiki/Planck_constant) *h* exactly to 6.62607015×10−34 J⋅s (J = kg⋅m2⋅s−2), given the definitions of the metre and the second.”, while that for Kelvin is, “The kelvin is defined by setting the fixed numerical value of the [Boltzmann constant](https://en.wikipedia.org/wiki/Boltzmann_constant) *k* to 1.380649×10−23 J⋅K−1, (J = kg⋅m2⋅s−2), given the definition of the kilogram, the metre, and the second.”
These are the only two, in the list of 7 which don’t help form an idea of what that quantity actually is.
In: Physics
There is a bit of debate in the interpretation of a unit in the first place. The whole thing of measuring is rather arbitrary. Historically we have always just used what’s convenient (hands, feet for length, stones for weight for example). In SI, we want to define very specifically what these things are, so that we can be precise about what we mean when we say kilogram or second or kelvin.
It’s quite natural to define things in terms of physical constants, since these are numbers that are pretty universal. If you found extraterrestrial life, you could explain to an alien exactly how a kilogram is defined using this language, and the *only* assumption is that physics has not changed.
For example, you mention the definition of a second, that’s very intuitive. Say “take the atom with 55 protons and 78 neutrons and start counting” instead of “ok come to earth and measure one full revolution of the sun then divide by 3600”. One of which is far more universal than the other, and also much more specific
The physical constants you mention actually are also quite intuitive as well. For example, the planck constant can be found by saying “take the atom that is just one proton and one electron and put it in a magnetic field, h is the difference between the lowest and second lowest energy”.
These are numbers that exist everywhere, and things that have “real” meaning, since they correspond to universal physical phenomena. Things like meters, kilograms, etc are arbitrary scales that don’t really have meaning on its own, so in order to give them meaning we use the physical constants to define them, in a super roundabout way
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