Send token when receiving BNB

I am compiling the ERC20Basic contract on the Remix, so I receive the token code and I can send BNB for this contract.

However, when I send BNB the function to send me the contract tokens that I created in ERC20Basic is in the DEX contract.

I do not understand how it is possible to do this sending since I compile only the ERC20Basic in Remix, and not DEX.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

interface IERC20 {

    function totalSupply() external view returns (uint256);
    function balanceOf(address account) external view returns (uint256);
    function allowance(address owner, address spender) external view returns (uint256);

    function transfer(address recipient, uint256 amount) external returns (bool);
    function approve(address spender, uint256 amount) external returns (bool);
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);


    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);

}


contract ERC20Basic is IERC20{

    string public constant name = "ByeSeel";
    string public constant symbol = "BYS";
    uint8 public constant decimals = 18;


//event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
//event Transfer(address indexed from, address indexed to, uint tokens);


mapping(address => uint256) balances;

mapping(address => mapping (address => uint256)) allowed;

uint256 totalSupply_ = 10000000 * 10 ** 18;

using SafeMath for uint256;

constructor() {
    balances[msg.sender] = totalSupply_;
}

function totalSupply() public override view returns (uint256) {
    return totalSupply_;
}

function balanceOf(address tokenOwner) public override view returns (uint256) {
    return balances[tokenOwner];
}

function transfer(address receiver, uint256 numTokens) public override returns (bool) {
    require(numTokens <= balances[msg.sender]);
    balances[msg.sender] = balances[msg.sender].sub(numTokens);
    balances[receiver] = balances[receiver].add(numTokens);
    emit Transfer(msg.sender, receiver, numTokens);
    return true;
}

function approve(address delegate, uint256 numTokens) public override returns (bool) {
    allowed[msg.sender][delegate] = numTokens;
    emit Approval(msg.sender, delegate, numTokens);
    return true;
}

function allowance(address owner, address delegate) public override view returns (uint) {
return allowed[owner][delegate];
}

function transferFrom(address owner, address buyer, uint256 numTokens) public override returns (bool) {
require(numTokens <= balances[owner]);
require(numTokens <= allowed[owner][msg.sender]);

balances[owner] = balances[owner].sub(numTokens);
allowed[owner][msg.sender] = allowed[owner][msg.sender].sub(numTokens);
balances[buyer] = balances[buyer].add(numTokens);
emit Transfer(owner, buyer, numTokens);
return true;
}

event Received(address, uint);
 receive() external payable {
 emit Received(msg.sender, msg.value);
}

}

library SafeMath {
function sub(uint256 a, uint256 b) internal pure returns (uint256) {
  assert(b <= a);
  return a - b;
}

function add(uint256 a, uint256 b) internal pure returns (uint256) {
   uint256 c = a + b;
   assert(c >= a);
   return c;
} }

contract DEX {

    event Bought(uint256 amount);
    event Sold(uint256 amount);


    IERC20 public token;

    constructor() {
        token = new ERC20Basic();
    }

    function buy() payable public {
        uint256 amountTobuy = msg.value;
        uint256 dexBalance = token.balanceOf(address(this));
        require(amountTobuy > 0, "You need to send some Ether");
        require(amountTobuy <= dexBalance, "Not enough tokens in the reserve");
        token.transfer(msg.sender, amountTobuy);
        emit Bought(amountTobuy);
    }

    function sell(uint256 amount) public {
        require(amount > 0, "You need to sell at least some tokens");
        uint256 allowance = token.allowance(msg.sender, address(this));
        require(allowance >= amount, "Check the token allowance");
        token.transferFrom(msg.sender, address(this), amount);
        payable(msg.sender).transfer(amount);
        emit Sold(amount);
    }

    receive() external payable {
    buy();
    }

}

In this case, I just want to send BNB and return my token, but I don’t know if I will compile the ERC20Basic contract or the DEX contract.

It seems like you want a crowdsale contract, maybe you can have a look at this contract:

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But in this case, how do I implement it? I’m new to this and I don’t know how to make an implementation with this sample code that you passed.
Do you have a tutorial?

Maybe you can have a look at this tutorial:

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