Model files in CI4

Sorry for the very late reply. I was busy with "life"...
Anyway, like ikesela said, use the model() function to load a model. It will return a shared instance by default. So you you don't need to use a global variable, this is PHP 4 era mentality. 

1)
Does this work in the subroutines in the libraries too?
I mean: in Library01.php methods I don't need to declare global $G_Mymodel01?

2)
I'm an old man, born with Commodore64's Basic. Maybe I'm not using objects in the best way...
But I can't imagine how to avoid global variables. My site is a game of strategy: the execute_move.php controller must load all the DB tables (80, more or less), then elaborate all orders, move units, solve combats, and so on; at the end, write the changes on the DB. I use 3 levels of subroutines, in 3 libraries. How can I do this, withot global objects?