I install Generator systems of various sizes for commercial and residential applications as a part of my buisness. A simple system is similar to what we have in our coach. Two sources of power directed thru a auto transfer switch powering a breaker panel.
In a residence, you can do this several ways. One way is to Install a manual transfer switch after the Utility Service & before the breaker panel, which then switches between a choice of Utility power and generator power.
Another way is to isolate the circuits you would like to have when power is lost to a seperate breaker panel . This emergency panel would be fed by a manual transfer switch which is powered either by the house power or generator power.
This second choice reduces the amount of circuits on the emergency power therfore saving you from needing a large generator.