Scalar

Assignment operator =

Not much to be said about Simple Assignment: of a scalar, It is a copy the memory to another address on the stack. No ownership is transferred.

Borrow operator &

rust-lang defines the scope as: “A scope is the region of source text where a named entity may be referenced with that name.”
To keep it simple know a scope is the text enclosed by a pair of braces {} or when the variable is last used.

At the end of the scope of b the ownership is returned to a.

fn main() {
    let mut a = 5;
    println!("{a}");
    {
        let b = &mut a; // the ownership of the memory of a is transferred to b.
        *b = 1;
        // a is not accessible because memory ownership is transfer to b
        // println!("{a}"); // !!! try to uncomment this line.
        println!("{b}");
        println!("{}", *b);
        // b goes out of the scope, so a regain the ownership.
    }
    println!("{a}");
}