Chosen Solution
Can i use a 6s SIM card tray in a 6s Plus? and vice-versa? for that matter, with the 6, 7, and 8, are the SIM card trays interchangeable between the base model and the Plus model (within the same phone family)?
No, the iPhone 6s SIM card tray cannot be used in the iPhone 6s Plus, 7, or 8. If you want a SIM card tray for the iPhone 6s Plus, you can get it here.
