How do I optimize the "Player Loop"?

I´ve read the documentation regarding Player Loop, but I don´t quite get it. It says:

“This class contains functions for interacting with the player loop in the core of Unity. You can use this class to get the update order of all native systems and set a custom order with new script entry points inserted.”

What class is that? Where and how am I using it?
I have big spikes in my profiler due to the PlayerLoop, but it doesn´t specify where is it or how to fix it.

Thanks

The PlayerLoop is the “heart” of a Unity game. You can never access it since it’s integrated into the UnityEngine. The profiler assigns everything inside your game under the PlayerLoop (and the Editors components under the EditorLoop). Therefore any spike “caused by” the PlayerLoop is actually caused by your own scripts,config, setup and things like that, including graphics too.

That is not true, you can make a custom player loop since Unity 2018.