The player loop is the configurable set of operations that make up the Unity Frame Loop. The default Player Loop comes with a variety of different subsystems, but the documentation is extraordinarily lacking.
Time.time is updated. [research needed]
Handles all scene loading. See SceneManager.
“takes the data that platform specific code queried and transforms it to be directly consumable by the Input class. It does stuff like combine all controller inputs into gamepad 0, calculates axis weights according to what is specified in the input manager, etc. For the new input system, it dispatches those events to managed code.” 1)