In GM, i need to know a few things

1 - you know how u can make a variable right? well, how do you make a global variable that is known to all objects (and not something like OBobject.variable) but something that doesnt need an object to exist or doesnt change or disappear when u switch from rooms? (example score, health, etc)

2 - Is there a way to change the game ID in the game? (so i can have different high scores tables, like 1 for the main game, maybe a different set of scores for different mini games, maybe one for battle mode or something)

3 - how do u tell game maker to open something from a directory? like in the game?