eli5: If absolute 0 is only theoretical, how do we know what it is in Celsius and fereignheit
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Absolute 0, or 0 K (Kelvin), is well defined. It is theoretical in that sense that no matter can achieve this temperature due to quantum mechanics, only become really close. There is no negative temperature in the Kelvin scale, thus absolute zero is at 0 K (hence the name).
1 K is defined to be the same as 1 °C, but the Celsius scale chooses a different point zero, which is where water freezes. Having the same scale, and water freezing at around 273.15 K (at 1 atm) makes absolute zero in the Celsius scale -273.15 °C. Fahrenheit chooses a different scale than Kelvin/Celsius and also a different point zero. But due to the same reasons as to why Celsius has negative temperatures, Fahrenheit also has absolute zero as a negative value.
As for the physics, only the definition of Kelvin with absolute zero at 0 K really makes sense. The scale in all cases is arbitrary, but we chose the scale of Celsius because it was widely adopted.
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