It’s mentioned in the night 5 phone call that the old animatronics came from the first incarnation of Fazbears, Fredbear’s Family Diner. They were kept and used for the third location (FNAF 1) because they were not fitted with ALL the new technology, thus they weren’t as dangerous. This is also why the toy ones were scrapped.
On a side note, it’s also very slightly implied that the guy who switched to the day shift after complaints may be the one behind the five kids being murdered after night 5. Supporting this idea is that on night 5, the Phone guy says he’ll “try” to get you into the day shift if there’s ever an opening. However, after he mentions the murders having happened, he says that the day shift is indeed available for one final birthday party.
As mentioned in the above image, Jeremy could have been the victim of the bite of ’87 that following morning, as he was tasked to stay close to the animatronics, who were aggressive towards adults after the Golden Freddy murder incident. Heck, their increasing aggression could also imply that this murder had been planned for a while, especially after seeing that lone endo skeleton occasionally.