Normally the fan have an over-run on a timer, so take a 'hard' permanent feed and 'switched' live, so they come on with the light, but of a period of time later.
Depending on how it is wired, often you will have all the required wires at the light fitting. Fan isolators then have three poles to isolate L, N and SW-L for maintenance on the fan.
Sometimes people put a second switch in, so one gives light only, the other light and fan, or mirror light and fan etc. So you dont have to have the fan on whenever the light is on.
More expensive fans, such as the S&P silent range, are quieter than others.
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