Multiplayer variables

Hello,
I am a bit confused about the handling of variables in multiplayer mode.

User case 1:
I have 2 players. Each player earns 10 points when they collect a coin. How do I communicate the 10 points to the other player?
Do I send a message? Do I use shared scene variables (if so, how)? Do I have to shared object variables (if so, how)?

User Case 2:
Are global variables automatically shared when they change value in either session?

User case 3:
I want to manage a timer that is always aligned between the two sessions. The timer starts at zero and goes up. In case a player takes a clock, both timers decrease by 10 seconds.

Sorry if I asked a lot of things. Reading the document is not clear to me.

Thank you,
J

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