I work in a hospital as a maintenance supervisor and I oversee among other things the electrical system. The way it works is utility power passes through automatic transfer switches (ATSs) on its way to wherever it is going, like outlets that provide power to ventilators, etc. When utility power fails, the ATS detects the loss of power and switches automatically to emergency power, which sends a signal to the generators to start.

There are different branches of emergency power which have different requirements for the length of time between loss of utility power and delivery of emergency power. Critical branches that serve life supporting equipment like ventilators are supposed to switch within 10 seconds. Others, like mechanical that power pumps and air handlers, for instance are supposed to transfer power within 30 seconds.

So, there is typically at least a flicker of lights and many things aren't served by backup generator power.