Once the scene is as safe as you can make it, the order is the same as any other trauma scene.
Catastrophic bleeding first. Blood leaving the body faster than anything else can matter. Direct, firm pressure with whatever is to hand, held without lifting to look. If a limb is bleeding beyond control and you have a tourniquet and know how to apply it: high, tight, and write the time. Guidance is explicit that a tourniquet is not loosened by a first aider once applied.
Airway. An unresponsive person's airway is at risk from their own tongue. Head tilt and chin lift unless you suspect a neck injury; where you do, a jaw thrust. In a vehicle, in practice this often means supporting the head in the position it is in and keeping the airway open there.
Breathing. Look, listen and feel for no more than ten seconds. Not breathing normally, or only gasping, means start CPR.
Compressions. Centre of the chest, 5–6 cm deep, 100–120 per minute, allow full recoil. Continue until the emergency service takes over or the person shows clear signs of life. If a defibrillator is available, use it as soon as it arrives and follow its spoken instructions.
Circulation and shock. Pale, cold, clammy, confused, fast shallow breathing. Keep them still, keep them warm, keep talking to them. Nothing to eat or drink.
Trapped casualties. If somebody is trapped, your job is to keep them conscious, keep them warm and keep their airway open where you can reach it. Extrication is for the service with the equipment; pulling on a trapped person causes injuries that were not there before.
Somebody who says they are fine. Adrenaline hides serious injury for twenty minutes or more. A casualty who walks away is still a casualty and should be seen.
Source. European Resuscitation Council Guidelines 2021 — Basic Life Support and First Aid, https://cprguidelines.eu/ ; ILCOR consensus on science, https://www.ilcor.org/
This is reading, not training.