Implicit conversion from Arc<ConcreteType> to Arc<dyn Trait>

How this code works in (1):

use std::sync::Arc;

trait MyTrait {
    fn f(&self) -> usize;
}    
impl MyTrait for String { fn f(&self) -> usize { self.len() } }

fn f(value: Arc<dyn MyTrait>) {}
fn main() {
    let p: Arc<String> = Arc::new("aaa".to_string());
    f(p);//1
}

?

The initial p is smart pointer to String structure, while Arc<dyn MyTrait> is smart pointer to String plus pointer to vtable. So how implicit conversion from Arc<String> -> Arc<dyn MyTrait> works, where pointer to vtable stored?

It's right next to the pointer to the data. Arc<dyn Trait> (and generally, all pointer-like types to all dynamically-sized types) are wide pointers, with metadata (vtable or length information) stored along with the "real" pointer.