How a solar collector, twin-coil cylinder and backup heat source fit together, and how solar thermal differs from solar PV.
What the collector does
A roof collector captures the sun's heat rather than making electricity: an absorber inside it warms a fluid, usually water with antifreeze, that a pump circulates down to a coil low in the cylinder. That fluid gives up its heat to the stored water and returns to the collector to be warmed again; the two never mix, because the collector circuit is sealed. This is solar thermal. Solar photovoltaic (PV) panels are a different technology that turns sunlight into electricity instead, which can then run an immersion heater through a diverter, but that is not the same as a solar thermal collector heating water directly. Flat-plate and evacuated-tube panels are the two common designs of solar thermal collector; both capture the sun's heat, just built differently.
Why solar thermal needs a backup heat source
Collector output follows the sun: it varies with season, weather, cloud and day length, and falls to nothing after dark, when the controller stops the pump rather than circulating water through a now-colder panel. Hot water demand does not vary with the weather, so a backup heat source, typically a boiler or immersion heater, is what guarantees hot water whenever solar input alone cannot meet it.
A twin-coil cylinder and how the controls prioritise heat
A solar system normally needs a cylinder with two coils. The solar coil sits low in the cylinder, so it can pre-heat the coolest water at the bottom and, on a good day, warm the whole store; the backup coil, or an immersion heater, sits higher, topping up only the amount the sun could not provide. Because solar heat is free and the backup is not, the controls let the collector heat the store as far as it can first, calling the backup only for the remaining shortfall; running the backup first would leave the cylinder already hot with nowhere for the free solar gain to go.