While there are indeed more then one legimitate and perfectly code compliant methods, my preference as an engineer was to use the simpler method where the Transfer Switch selected EITHER the Utility to feed the homes exisitng Main Panel ORRRRRRRR the Genset fed the exisitng Main Panel.....Its so easy to switch ON ONLY the loads you wish to use and the others OFF when the genset is the selected source of power if it lacks capacity to power everything.
The Main Panel receives power FROM THE TRANSFER SWITCH (NOT the Utility as normal) ,,,,,,The Switch is wired to TWO possible sources, the Utility orrrrrrrrr the Genset. PIECE OF CAKE TO WIRE AND USE AND no rewiring or reconnecting some loads one place or another ALL LOADS ARE ALREADY FED FROM THE MAIN PANEL AND IN PLACE simply turn a few off as/if necessary.
ALSO you only need to use a TWO POLE TRANSFER SWITCH and you DO NOT have to switch the Neutrals PROVIDED the Genset isnt configured as a Seperately Derived Source!!!!!!!
Still there are cost considerations and convenience and other factors where another method may work best in ones own particular situation SO TAKE A LOOK AND SEE and consult local authority and utilitly and professional electricians and engineers!!!!!!!!!
Your money your situation your choice, we report now you decide