(Weird how a brand-new iPhone doesn’t have a feature the two-year-old iPhone 12 Pro does.).
Even if Apple wanted to add this feature to non-Pro iPhones, it likely couldn’t—the measure feature is hardware-based, relying on the LiDAR sensor built-into the cameras of the Pro iPhones.
One of the usages for such a sensor is the ability to instantly measure a person, so long as the person is standing in front of the camera from head to toe.
To measure a friend with your iPhone, open the Measure app and frame your friend in Measure’s camera viewfinder, making sure their entire body is visible.