Understanding Military Hierarchy: Who's in command?

Okay so I'm trying to get a solid understanding of military structure and something I've found confusing is the command system. Can someone please explain to me which branch of the military typically has command authority?

The military's command structure is a bit complex because it is divided among several components and has various levels of hierarchy. Each branch of the military -Air Force, Army, Navy, Marine Corps and Coast Guard- has its own command structure which comprises five major components: Enlisted members, Non-commissioned officers, Company Grade Officers, Field Grade Officers, and General Officers.

At the individual unit level, usually a company or squadron, officers who are Lieutenants or Captains serve as Company Commanders or Squadron Commanders, while enlisted members with ranks of Sergeant or above serve in lower leadership positions. A battalion or squadron is commanded by someone with the rank of Lieutenant Colonel or Commander. Regiments or brigades will be led by a Colonel, while divisions, Corps, fleets, or wings are led by Generals or Admirals.

However, the overall military command doesn't fall under a specific branch per se, as it's not associated with the Army, Air Force, or any other specific military branch. The highest military command in the United States is the President, who acts as the Commander-in-Chief. Below the President, the Secretary of Defense, a civilian, advises on military policies and affairs. Beneath them are the Chairman and the Vice Chairman of the Joint Chiefs of Staff who provide military advice to the President, Secretary of Defense among others. The Joint Chiefs of Staff includes the highest-ranking officer from each branch of the military, and each has equal status with the others.

Furthermore, there is the Unified Combatant Command which includes combatant commands responsible for different regions and functions. These commands are led by four-star generals/admirals from the various military branches, thus making it branch neutral. A famous example is the US Central Command (CENTCOM), responsible for the Middle East and Central Asia.