Yes, the heating-bill is variable. It is also a notorious bone of contention in rented flats, precisely because there are various models of how the landlord tries to apportion the total heating costs of a season. Some time in Spring or Summer, a caclulation is made. Some landlords use "number of rooms", others use "number of persons" others use "number of square metres" and they divide out the building's heating costs according to that formula. Then, you just have to pay in whatever difference there is, or, in a good year, you get a refund.
Only very few buildings have metres on each radiators which allow a true "use per flat". Only in such a flat would you pay less if you used less. To landlords using the other systems of calcualtion, even if you were away most of the winter, with your radiators set only on the minimum (frost-prevention) level, you would still have to pay your share.
At a first glance, this may seem unfair. However, each flat's consumption affects the neighbours. If someone turns their heating up full, then naturally the adjacent flats will be, at least in part, warmed by that, too. Likewise, if someone turns their own heating down low, then they would not be co-heating for their neighbours, and therefore the neighbours would be more likely to [have to] turn the radiators up.
Add to that the fact that the flats on the ground floor are right on the cold earth (or cold cellar) and the flats on the top floor right under the snow. The flats in the middle are somewhat insulated, and may need to use less heating because they profit from their neighbours upstairs and downstairs.
In any case, this topic can be endlessly discussed, and it can be a really wise step, psychologically, just to set aside some hundreds of Francs, from the start, in case, in Summer, you are sent a bill.
Nebenkosten can also include non-variable cost such as the cleaner of the stair-well. Historically, many flats made it a duty that the tenants took turns to clean the stairs. This kept the costs down, and that was very important during and after the World War II, when many flats in Switzerland were built. As society has become wealthier and busier, tenants have become less willing to do this work, so that landlords are more inclined just to pay a cleaning-service and charge this to the tenants.
Electricity is metered per flat. Usually somewhere in the cellar, there is an electrical meter. When you move in, it is a good idea to check that the caretaker or the representative of the electricity company comes by to take the meter reading. That way, it is clear that - up till then - the amount used is the responsiblity of the tenant before you. From that reading, (which should be very close to the day you move in) you pay a monthly or quarterly (depending on the local rules) electricity bill. It is directly according to your own consumption, i.e. use more = pay more. However, electricity is absurdly cheap in Switzerland (though this, too, like everything else, can vary from area to area), so that for tenants of middle-class homes it is not usually felt to be a major expense, when set in relation to rental and other fixed expenses.