You are playing on save A and then create a save B, the game will now autosave on save B, and you can load Save A to return to the part that you was, then you can overwrite the save B if you want to choose a different option in a scene, or create a save C.
The game will always autosave on the last file slot that you saved.
Without autosave, the game would be very complex in making a save system, each scene has with own collectible items, that need to be saved, the world map has too locals that you already visited to save.
This was the best that I could do with my knowledge.