Presumed, while it is technically possible it is not recommended. Transfer switches have mechanical interlocks so that you can not have line power and generator power at the same time. If by chance you hook up the generator and forget to switch off the mains or someone else switches on the mains your generator will be back feeding the power lines. The power will go back out through the transformers at 7200 volts, this is can be lethal to line workers. If no one gets shocked the power will come back on and fry your generator.

There are generator transfer switch set ups that are automatic, line power goes off, generator comes on. When the power comes back on the generator shuts down. These systems are generally expensive.

If you are using a small transfer switch it will not disconnect your whole house from line power, leave a light on in a circuit that is still on line power and when the power come back on so will the light. It is also advisable to turn off all major appliances that are still on line power. HTH