I'm asking about the general method of saving large amount of data. If there are hundreds of objects and some got changes with their properties like coordinates, life and status, how do other games handle the saving, especially the ones with autosaving feature? As I wrote in the original post, do they save the whole thing? If that's the case, wouldn't the size of the save be too big for periodic autosaving, e.g slowing down the game for each instance of saving?
Depends on whats the game's minimum requirements... that's an optimizing issue, that you shouldn't worry about until after you have the game done. Then you can choose what to save or not. If you want that halo-style auto saving with everything saved pixel perfect, you can understand why a lot of games don't save the entire game state like that, if you're worried about performance.
You either lean on performance, or lean on quality. It's not that bad tho, and you can sneak in stuff to a savefile throughout the game to stop from a giant bulk dump. like health and status only need to saved when changed, other than x,y, coordinates most things can be saved to a temp file that's renamed at save.
I still see major games that fail at saving states properly, it's really the trickiest part, what should be in the save and what isn't, because it's one of the biggest factors in performance, when done mid game. This is why you see so many games have a seperate save window.