Question Changing scenes without closing them

So I'm making a game where you will get flashbacks. And those flashbacks will be in a separate scene called "Flashbacks "from the main game scene. I already implemented system of changing scenes using this video.

but I want to make it so when I come back to my main scene nothing has changed. Doors there were opened are still opened, items that you have picked up are still picked up etc. How can I make that?

 

Two ways: Additively load the flash-back scene on top of the existing scene.

Or, implement some form of data permanency system, so the original state can be built when you load back into the previous scene. Aka, a save system.