Contract to demo fallback function and receive function.

DemoFallbackFunctions.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;

contract DemoFallbackFunctions {

    event Log(string typeOfFunction, uint gas);

    // Fallback function must be declared as external.
    fallback() external payable {
        // send / transfer (forwards 2300 gas to this fallback function)
        // call (forwards all of the gas)
        emit Log("fallback", gasleft());
    }

    // Receive is a variant of fallback that is triggered when msg.data is empty
    receive() external payable {
        emit Log("receive", gasleft());
    }

    // Helper function to check the balance of this contract
    function getBalance() public view returns (uint) {
        return address(this).balance;
    }
}

contract TestFallbackFunctions {

    address payable contractAddress;

    constructor(address payable _addr) {
        contractAddress = _addr;
    }

    function callReceiveFunction() public payable {
        contractAddress.transfer(msg.value);
    }

    function callFallbackFunction() public payable {
        (bool sent,) = contractAddress.call{value: msg.value}("abcd");
        require(sent, "Fallback function call failed");
    }
}