It would depend on make and model, I hate the word digital and smart because they can mean so many things, in theory digital is off/on so with electric heaters they are all switched off/on, but a standard thermostat will wear out quickly if it turns off/on every few minutes, so using solid state relays allows the manufacturer to reduce the time between the off/on cycles without the contacts failing so reducing the hysteresis, however there is no laid down description of digital so no way to work out what you have.
Many people seem to call a dial analogue and up/down buttons and a LCD display digital and it has nothing to do with how the heater works but only refers to the human machine interface.
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