There have been moments where I've quit immediately after an autosave icon appears and disappears, and reloaded to find I've been sent back in time to another autosave location instead. However, from my experience, this doesn't always appear to be entirely accurate. Theoretically, you'll know when an autosave has occurred by the small white icon that appears briefly in the top-right of your screen: It appears to autosave whenever you're approaching or entering a new location of the map, such as an Anti-Aircraft Site, along with certain story and mission beats. All saving is done through autosaves, and the game chooses when to autosave.
It is not possible to manually save your game in Far Cry 6.
