On mine, the electrician just mounted a box on the house near the generator, and put an interlock on my existing panel.
I think the whole deal was a couple hundred bucks.
Oddly, my well pump runs off a separate box (main house service), which I didn't know at the time.
So, even if the gen is supplying the house, it's "after" the well pump.
Not a big deal for me, though I wish I'd known that; I just keep water (5-gal bucket) to refill the toilet and keep a few gallons of drinking water.
My main service would be a pain to get to in a decent snowstorm anyway,