u/PokemonJimbob

It starts on Night 1 during the salvage section. In it, you will need to find an old toy(a race car) lying around somewhere(probably somewhere on the stage that Withered Freddy is on). After collecting it, you can salvage as normal.

Night 2 has no differences until you beat it. Outside, you meet Mary Schmidt(from Candy's 1 or after). She'll ask about a toy that you picked up and ask that you help her find more of them, saying she'll explain more if you help.

At the Salvage, you can find the 2nd toy, a rocking horse, in the Parts and Service room(which you will need to unlock like in the Salvage ending). However, holding onto the rocking horse with a tight grip is the Rat, who you will need to salvage to collect the toy. Night 3 onward will have the Rat appear. The Rat will show up at your window and you need to shine your light on him. Not to send him away, but to see which eyehole in the suit has an eye in it(semi-referencing the final frame of their FNAC 1 jumpscare, image included). You will need to close the door that the eye is on(if the eye is on OUR left we close the left door for example).

Meeting Mary again, we tell her about the "creepy rat suit" that was holding the toy. Mary is surprised and explains that the toys are full of precious memories, and that something seems to want to prevent them from being collected. She asks that we continue collecting as they're important to her. The third toy, a duck toy, can be found in the office by the box of parts.

Night 4 has nothing new added. Meeting Mary again, she'll congratulate us on finding another one before we ask her a question. The guard we chose will ask her why she doesn't just go after the toys herself. She'll respond by saying that she's scared, she feels like something is out to get her and is using them as bait. Despite this, she asks us to keep collecting them.

The fourth toy, a toy robot, is in the Crying Child's bedroom. It will be sticking out from under the bed with a suit holding onto it. Another animatronic we have to salvage. This time it's the Cat.

On Night 5, the Cat will be active. He will already be in the office and will slowly creep out of the far ends of the office. We just have to look at him for him to creep back into the corner and out of sight. If he gets too far out of the corner, he'll take our flashlight and kill us if we look at the corner he was coming out of. He'll be standing close to the door, but not close enough that looking at the door kills us.

After beating the night, Mary is told about the Cat and she says she has a bad feeling about things. She brushes it off to say that there's one more toy left to find and she might have an idea of its location. She'll tell you about Candy's Burgers and Fries. Though she also says that we may need to perform a salvage if we want the trip there to look worth it to the Boss.

At Candy's we'll find all the Candy's 1 animatronics there(minus the Rat) and all in their proper places(except for Candy who can be standing in the office). We can find the final toy, a teapot, in the room that the Rat usually comes out of. Though we'll need to unlock it. We can find Shadow Candy hidden hidden in the ticket booth window, though not on the office side of it. He'll tell us to look for a picture somewhere and to click on the one usually locked in the room we're trying to get to. Upon doing so, we can grab the teapot and leave.(And instead of Psychic Friend Fredbear being at the entrance and being the one we talk to so we can leave, it is instead the Origami Cat, who won't let us leave until we grab both the Teapot and the salvages. (I'll share my ideas for each of their mechanics in the comments)

On Night 6 we have an intro scene where we get a call from Mary thanking us for all our help. She starts talking about how she won't let our hard work go to waste before we hear a distorted, backwards music box melody. Suddenly, a reverse puppet(Vinnie's original name and design) will drop down on the other side of the office window. They will then flicker between all three designs(reverse puppet, Monster, and non-monster) before settling on the non-monster Vinnie design(image included). He recognizes Mary's voice and says that we're "in cahoots" with her. He says that he recognizes our role in assisting her and that our role has come to and end. And once the curtains have fallen for us, he'll be going after "that disrespectful little girl who ruined [his] life"(despite her being an adult, this is more of a reference to how Mary was young when she got him arrested and went against Monster Vinnie). He will then fly upwards on invisible strings as the night begins. It will have different phases divided by time.

12 AM: Only Vinnie and the animatronics we salvaged from Candy's are active. Vinnie will appear in the window and we have to shine our light in his face as he moves around. This doesn't last anywhere NEAR as long as in FNAC 3. It's only a few seconds. Mary will call us every hour to inform us of what's going on("Keep an eye on your window! He doesn't like bright lights being shined in his face...though, I don't think anyone really likes that..."
1 AM: The Rat activates("The Rat looks like they've begun moving. Don't let your guard down!"
2 AM: The Puppet activates and Vinnie gets a new mechanic. He'll sometimes let out a laugh and you'll have to find what camera he's on and quickly flash him. He'll let out an agitated grunt and flee.("He's moving throughout the building now! Listen for his laugh and then flash him on the cameras! And keep an eye on that other puppet thing")
3 AM: The Cat activates("Now the Cat has started moving. Remember to watch the corners of your office!")
4 AM: Another new mechanic for Vinnie. Another camera mechanic, but this time, no looking at him. If you look at him too long, the cameras will crash and will need to reboot. Trying to look at them while they reboot will just have an image saying "My mistake. Your Problem" To differentiate when to look and not look, there will be two differences. 1: There will be no sound cue. 2: He'll have eyes(image included) when you shouldn't look for him("He really likes messing with us. Don't look into his eyes on the cameras...if he has them that is..."(then in a quieter voice)"How would a puppet even have eyes?")
5 AM: Everyone else activates but at lower AI levels(around half of what they normally are)("Final stretch! Just hold out for a little longer! And watch out! All the other animatronics look like they've activated!"
Upon beating the night, there will be a cutscene.
Vinnie will show up at the door, saying he's done letting us go off the script he made. He'll start to reach in...before it gets closed ON him. we can hear a bit of a cracking sound. We let him stay there before we open the door and Mary will enter. She'll say she saw the whole thing through a window(which is how she was able to help us through the phone call. She'll start to thank us for dealing with Vinnie...before Vinnie gets up, looking damaged, and starts to choke her. She'll struggle for a few seconds before managing to push him off where he'll hit the back of his neck on the corner of the office desk. We'll hear another crack and he'll slump to the floor while his head dangles upside down. The post-credits image will have her fist-bumping Jeremy, shaking hands with Mike, hugging Vanessa, and just kinda awkwardly giving Fritz a thumbs-up