How does Client-Server calls work in online games?

Suppose I have simple Breakout game which is an online game running on browser. I am updating all sprites continuously. My question is, how do they handle server calls? Do they make server calls during each sprite update? or it happens only at client side? If at client side, how do I retrieve my score when connection is lost? How does thing work in multiplayer environment.

Trying to understand the concept though a sample code would be helpful.