Solidity – How to Create a DEX for an Already Generated ERC-20 Token

I want to create dex (buy – sell) function and deploy it on another generated ERC-20

I have code like this

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 = "ERC20Basic";
    string public constant symbol = "ERC";
    uint8 public constant decimals = 18;


    mapping(address => uint256) balances;

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

    uint256 totalSupply_ = 10 ether;


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


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);
    }

}

and problem is here in Contract Dex

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

it only works when this contract generate a token

i want to create something like

constructor() {
            token = 0xdAC17F958D2ee523a2206206994597C13D831ec7; //usdt token address
        }

after than i can deposit usdt on this contract and buy or sell it like a dex

but i cant find a way to do this