Understand the principle before the equipment
HVAC equipment usually operates as a sequence rather than all components starting simultaneously. The thermostat creates a request, then equipment controls decide which components should operate and in what order.
Safety inputs can interrupt the sequence if required conditions are not met. That is why a furnace may start an inducer but never light burners, or an air conditioner may run the indoor blower while the outdoor unit remains off.
Knowing the normal sequence helps organize troubleshooting observations without bypassing controls.