Example contract using delegateCall

delegateCall

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

contract Storage {
    uint public num;
    address implementation;

    constructor(address _implementation) {
        num = 0;
        implementation = _implementation;
    }

    function setNum(uint _num) public {
        (bool success, ) = implementation.delegatecall(
            abi.encodeWithSignature("setNum(uint256)", _num)
        );
        require(success, "Error while delegating call");
    }

}

contract Implementation {
    uint public num;

    function setNum(uint _num) public {
        num = _num;
    }
}

calling using interfaces

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.7.0 <0.9.0;

interface IImplementation {
    function setNum(uint _num) external;
    function num() external view returns (uint);
}

contract Storage {
    uint public num;
    address public implementation;

    constructor(address _implementation) {
        num = 0;
        implementation = _implementation;
    }

    function setNum(uint _num) public {
        IImplementation(implementation).setNum(_num);
    }
}

contract Implementation {
    uint public num;

    /**
     * @dev Sets the `num` in the Implementation contract's own storage.
     */
    function setNum(uint _num) public {
        num = _num;
    }
}