SIM card location iPhone 6s Plus

Find the SIM tray on the right side:

• iPhone SE (1st, 2nd, and 3rd generation)
• iPhone 8 Plus
• iPhone 8
• iPhone 7 Plus
• iPhone 7
• iPhone 6s Plus
• iPhone 6s
• iPhone 6 Plus
• iPhone 6
• iPhone 5s
• iPhone 5c
• iPhone 5
• iPhone 4s
• iPhone 4 (GSM model)

It's right there in the linked article above