For any subtype of Number, zero(T) is just a shorthand for convert(T, 0). See: https://github.com/JuliaLang/julia/blob/6a23e234e6cc5b4361b5f88614a9ed423dc2c12a/base/number.jl#L238
And for any type T, if you haven’t defined a constructor T(x), then it will also fall back to convert(T, x). See: https://github.com/JuliaLang/julia/blob/6a23e234e6cc5b4361b5f88614a9ed423dc2c12a/base/sysimg.jl#L114