How does Go achieve runtime polymorphism using interfaces?

type Shape interface {
    Area()      float64
    Perimeter() float64
}

type Circle struct{ Radius float64 }
func (c Circle) Area()      float64 { return math.Pi * c.Radius * c.Radius }
func (c Circle) Perimeter() float64 { return 2 * math.Pi * c.Radius }

type Rectangle struct{ W, H float64 }
func (r Rectangle) Area()      float64 { return r.W * r.H }
func (r Rectangle) Perimeter() float64 { return 2 * (r.W + r.H) }

type Triangle struct{ A, B, C float64 }
func (t Triangle) Area() float64 {
    s := (t.A + t.B + t.C) / 2
    return math.Sqrt(s * (s-t.A) * (s-t.B) * (s-t.C))
}
func (t Triangle) Perimeter() float64 { return t.A + t.B + t.C }

// Polymorphic function — works on any Shape
func printShapeInfo(s Shape) {
    fmt.Printf("%T → area=%.2f perimeter=%.2f\n",
        s, s.Area(), s.Perimeter())
}

shapes := []Shape{
    Circle{Radius: 5},
    Rectangle{W: 4, H: 6},
    Triangle{A: 3, B: 4, C: 5},
}
for _, s := range shapes {
    printShapeInfo(s) // dynamic dispatch via itab each iteration
}
// main.Circle   → area=78.54 perimeter=31.42
// main.Rectangle → area=24.00 perimeter=20.00
// main.Triangle  → area=6.00  perimeter=12.00