(Sorry if this shouldn't be asked here, I'm unsure if it's appropriate based on what's allowed here.)
I'm not developing a game, but I'm very curious about this. I've played some large open-world games recently (Dishonored, for example), and it boggles my mind how much information must be stored for save files. If I pick up a bottle from where it spawned and throw it, then save while it's in mid-air, the save file needs to contain a) what type of bottle it is, b) where it is right now, c) what orientation it's currently in, and d) what its current trajectory is. This is the case for every non-fixed asset in the current level. Pathing NPCs have to have their current state tracked as well.
So, in general, how do games accomplish this? I'm sure the implementation varies, I'm not asking for any specific game, but the overall concept must be similar across games. Even more impressive to me is that quick saves often seem to be instant, with no delay whatsoever after pressing the key.