  Since the seminal work of Keating and Snaith, the characteristic polynomial
of a random Haar-distributed unitary matrix has seen several of its functional
studied or turned into a conjecture; for instance:
  $ \bullet $ its value in $1$ (Keating-Snaith theorem),
  $ \bullet $ the truncation of its Fourier series up to any fraction of its
degree,
  $ \bullet $ the computation of the relative volume of the Birkhoff polytope,
  $ \bullet $ its products and ratios taken in different points,
  $ \bullet $ the product of its iterated derivatives in different points,
  $ \bullet $ functionals in relation with sums of divisor functions in $
\mathbb{F}_q[X] $.
  $ \bullet $ its mid-secular coefficients,
  $ \bullet $ the "moments of moments", etc.
  We revisit or compute for the first time the asymptotics of the integer
moments of these last functionals and several others. The method we use is a
very general one based on reproducing kernels, a symmetric function
generalisation of some classical orthogonal polynomials interpreted as the
Fourier transform of particular random variables and a local Central Limit
Theorem for these random variables. We moreover provide an equivalent paradigm
based on a randomisation of the mid-secular coefficients to rederive them all.
These methodologies give a new and unified framework for all the considered
limits and explain the apparition of Hankel determinants or Wronskians in the
limiting functional.
