The compressor.

The compressor is the pump. It takes low pressure refrigerant gas coming back from the house, squeezes it into a hot high pressure gas, and sends it out to the condenser coil to dump that heat outside. Everything else in the system exists to serve it.

It lives in the outdoor unit and it is the thing making the low hum when the system is running.

Why it matters out of proportion

A compressor is the most expensive single component in a residential system by a wide margin, and replacing one is often close enough to the cost of a new system that the conversation stops being about repair.

That is worth knowing because almost everything that kills a compressor is cheap to prevent:

Liquid coming back. Refrigerant is supposed to arrive as gas. When a coil is iced over it does not all boil off, and liquid gets pulled into a pump built for vapour. This is why running a frozen system is the expensive choice.

Running hot. A condenser coil packed with monsoon dust cannot reject heat, so pressures and temperatures climb and stay there, all summer.

Starting against a dead capacitor. A motor that tries and fails to start draws locked rotor current. Doing that repeatedly is hard on the windings.

What this business can and cannot do

No job here can go over one thousand dollars in labor and materials, and a compressor replacement is well past that. So if the diagnosis lands on the compressor, what you get is the readings, the photos, a plain explanation, and a referral to a licensed Arizona contractor. No fee and no markup on the handoff.

There is an upside to that: nobody here has anything to gain from telling you the compressor is gone. That is the entire reason a second opinion from here is worth having.