I want to know how to instantiate an existing contract deployed into the blockchain using its address. e.g:

contract A {
    funtcion f1()
    {}
}

Deployed into the blockchain, and in contract B I want to call the function f1() and get its return. e.g:

contract B {
    address contrac_A=0x123456;
    
    //call f from A
}

How should I call it using the address of A?

I want to know how to instantiate an existing contract deployed on the blockchain using its address. e.g:

contract A {
    function f1()
    {}
}

A is deployed on the blockchain, and in contract B I want to call the function f1() and get its return. e.g:

contract B {
    address contrac_A=0x123456;
    
    //call f1 from A
}

How should I call it using the address of A?