The package unit.
Everything in one cabinet, usually on the roof. Common on Valley ranch houses, and a different job to work on.
A package unit puts the whole system in one cabinet: compressor, condenser coil, evaporator coil, blower, and often a gas heat section. It sits outside, usually on the roof, and ducts drop straight down into the house.
A split system does the opposite. The compressor and condenser coil are in a unit outside, the blower and evaporator coil are in an air handler inside, and refrigerant lines connect them.
Why Phoenix has so many
Low-slope and flat roofs, which a lot of 1950s and 1960s Valley ranch houses have, make a rooftop package unit the obvious installation. South Scottsdale, big parts of central Phoenix, and plenty of Tempe north of the 60 are full of them.
It also means no indoor equipment eating a closet, which on a house of that era matters.
What is different when it breaks
Mechanically, very little. The same capacitor, contactor, fan motor and coil failures turn up, and a package unit still has a drain that plugs.
What changes is access. Getting to a roof unit means a ladder, tools carried up, and working on a surface that is well over 150 degrees on a summer afternoon. That is genuinely a different job from kneeling beside a unit in a side yard.
Here that is handled by having roof and attic work as its own row on the price list rather than as a surcharge added once somebody is already up there. You see the number before you book.
One thing worth checking yourself
Package units have a drain that terminates on the roof, and roof drains plug like any other. A unit that shuts itself off with no other symptom is worth checking for a tripped float switch before anything else.
Do not go on a Phoenix roof in July to check it.