  (Abridged) We calculate the full stellar-structural evolution of donors in AM
CVn systems formed through the WD channel coupled to the binary's evolution.
Contrary to assumptions made in prior modelling, these donors are not fully
convective over much of the AM CVn phase and do not evolve adiabatically under
mass loss indefinitely. Instead, we identify three distinct phases of
evolution: a mass transfer turn-on phase (during which the orbital period
continues to decrease even after contact, the donor contracts, and the mass
transfer rate accelerates to its maximum), a phase in which the donor expands
adiabatically in response to mass loss, and a cooling phase beginning at
orbital periods of approximately 45--55 minutes during which the donor
contracts. The physics that determines the behaviour in the first and third
phases, both of which are new outcomes of this study, are discussed in some
detail. We find the overall duration of the turn-on phase to be between $\sim
10^4$-$10^6$ yrs, significantly longer than prior estimates. We predict the
donor's luminosity and effective temperature. During the adiabatic expansion
phase (ignoring irradiation effects), the luminosity is approximately
$10^{-6}$--$10^{-4} L_\odot$ and the effective temperature is approximately
1000--1800 K. However, the flux generated in the accretion flow dominates the
donor's intrinsic light at all times. The impact of irradiation on the donor
extends the phase of adiabatic expansion to longer orbital periods and alters
the donor's observational characteristics. Irradiated donors during the
adiabatic phase can attain a surface luminosity of up to $\approx10^{-2}
L_\odot$. We argue that the turn-on and cooling phases both will leave
significant imprints on the AM CVn population's orbital period distribution.
