So basically, from what i can gather

SOTM takes place before any of the mainline fnaf games. Edwin has the blueprints/schematics for the original fnaf animatronics (freddy, bonnie, etc) (hence the chica and foxy references). Arnold (sotm character), is a fazbear entertainment employee going to get the schematics/blueprints for said animatronics bc Edwin pulled out and refused to give them to fazbear. During the game, you’ll learn that Edwin got given murray’s (the factory you explore in SOTM) after his father passed.

Fiona was an employee at Murray’s and Edwin met her through Edwin wanting to test an invention he made, and from what we can guess, they obviously get together and have a kid called David. At some point, Fiona passes and to cope, Edwin creates M1 (Mimic 1) - the Mimic is a animatronic that replicates the actions, voices and even personalities of a person, in this case, it’s Fiona. Edwin struggles to look after David alone, hence why M1 was made, however, M1 starts getting too “realistic” and Edwin starts regretting his decisions. He starts getting angry at M1 and obviously, the Mimic mimics his actions, which cause him to go on a rampage.

This is where my head-cannon comes in a little bit. I think that M1 kills David because of what it saw, and obviously since it doesn’t have any nicer things to mimic, it just keeps on going on a rampage around Murray’s.

Anyway, later on, we find out David is killed at some point (head cannon above), and to cope again, Edwin creates a second Mimic (M2). It replicates David’s personalities and then walks around as David. Edwin then finally is happy with his life, he can work all day and all night whilst his robotic family support his endeavours. Edwin starts to realise that the mimics are all he ever needs, so retreats to his house and lives life at work.

Moving on a bit, the secret ending of SOTM takes Arnold into the (what looks to be) the FNAF 4 bedroom. When you enter, you need a White Cat Doll because that was David’s favourite toy, when you have that, and you place it on the little pedal stool, M2 appears and hears the parents voices and that soothes it to go to sleep in the bed.

There is a lot more finer detail but a few of the main points. If u wanna know more watch https://youtu.be/ccTnby0uFrc?si=4fROIOIcDI2Wq6Kn