Selectors

…

2. For a value x of type I where I is an interface type, x.f denotes the actual method with name f of the dynamic value of x. If there is no method with name f in the method set of I, the selector expression is illegal.

type I interface {
	Greet()
}

type Person struct {
  Name string
}

func (p *Person) Greet() {
  fmt.Printf("Hello, %s", p.Name)
}

var I x
x = &Person{}

x.Greet // denotes the actual Greet method Person, which is the dynamic value of x

Quotes from The Go Programming Language Specification Version of August 2, 2023