About iPhone 8 camera

Use Portrait mode on your iPhone - Apple Support

Portrait mode uses the dual cameras on your iPhone 7 Plus, iPhone 8 Plus, and iPhone X to create a depth-of-field effect.

The dual cameras are necessary to measure the distance to subject using stereopsis. Because the two cameras are so close together,

you have to be very close to the subject, no further away than 2.5 meters, or the iPhone will complain.

You were mistaken. The cameras are different.

Compare these descriptions from Apple's Tech Specs page for the 8 and 8 Plus:

iPhone 8: "12MP camera"

iPhone 8 Plus: "12MP wide-angle and telephoto cameras"

iPhone 8: "ƒ/1.8 aperture"

iPhone 8 Plus: "Wide-angle: ƒ/1.8 aperture" and "Telephoto: ƒ/2.8 aperture"

iPhone 8: "Digital zoom up to 5x"

iPhone 8 Plus: "Optical zoom; digital zoom up to 10x"

iPhone 8: "—:

iPhone 8 Plus: "Portrait mode"

iPhone 8: "—:

iPhone 8 Plus: "Portrait Lighting (beta)"

Use Portrait mode on your iPhone - Apple Support

The rear cameras differ as the Plus has an extra lens that enables Portrait Mode (similar to 7 and 7 Plus).