except that you WILL have low level people as incident commander - to use a less sensitive term - someone is in charge

One guy on the scene - he IS in charge of the scene until someone more Sr shows up and relieves him - the hand off can even be verbal "this is what is going on" - that new person is now in charge, and the other person is now 'another worker' on the scene

A 2 man squad car shows up - One of the 2 guys - One's in charge, one isn't - the guy in charge is the incident commander - He's in charge of the incident

An Engine (use your example) shows up and is first on scene (or ONLY) - the Engine Boss IS the Incident Commander - the rest of the strike team shows - He says to the ST Commander "Here is the situation, here is what I know" - the second the ST Commander says "OK, I got it" - Guess what, the ST Commander is now the IC - and the former IC is now back to being the commander of his resource (aka an engine boss)

An IC is nothing but the Sr guy at the sight who is on duty, or if he doesn't WANT it, who he appoints

Been on a ship? Who's got the Con? That's the boss (IC) until someone of higher rank says "I have the Con" - That person then becomes the boss

Guess what? ICS wise (and I can understand it) - You're a good samaritan with say a first aid card, and you show up at an accident - YOU are IC - till the cop/EMT/FIRE/A MD/Nurse whoever with more experience shows up and says "OK, I'm here, I've got it" (not necessarly that formally) - THEY then have relieved you as IC - but SOMEONE is ALWAYS in charge - they call that guy, the Incident Commander - he may have NO formal skill, he may not even know he is an IC, or even know what an IC IS, but he IS

It's just a name given to whoever is in charge