Why does 0^0 in the macOS and iOS calculator give different results on different versions?
While 0⁰ is generally undefined, some branches of mathematics do explicitly define it as 1 because, as you can see, this is the value to which the function y(x) = xˣ converges at n=0.

Less formally, note that 0.50.5=0.707…; 0.20.2=0.725…; 0.10.1=0.794… and 0.010.01=0.955…. As you approach 0, the result will be approaching 1, which makes it quite logical and handy to define 0^0 as 1 in some cases.

Thus, none of these 3 results are incorrect per se and instead they all reflect different conventions on the value of this undefined expression.

There is a good Wikipedia article explaining the issue. See also Zero to the zero power – is 0⁰=1?.