How to separate access layer from business object?

Question

I am doing a task called , Expense Manager in linqpad using c#,asp.net core,npgsql, and PostgreSQL as database The objective of the task is to perform different operations on the database.
For Example: GetTransactions()-- to get all transactions performed.

like this there are 5 methods:
GetTransactionInfo(5); -- to get complete transactions information based on transaction id
GetIncome(startDate, endDate); -- total amount earned as income between dates GetExpense(startDate,endDate);
GetFilteredTransactionsList(startDate,endDate);

The database consists of 2 tables namely: Transaction, Account shown below in classes below with same names and columns.

These methods are all working perfectly as shown in the code here:

    void Main()
{
    DateTime startDate = new DateTime(2022, 01, 01);
    DateTime endDate = new DateTime(2022, 01, 30);
    TransactionAccess.MakeTransaction(new Transaction
    {
        Amount = 500,
        Date = startDate,
        Note = "Travel ticket",
    }, new Account
    {
        Account_Name = "shopping",
        Type = "expense"
    });

    TransactionAccess.CreateAccount(new Account
    {
        Account_Name = "shopping",
        Type = "expense"
    });

        TransactionAccess.GetTransactions();
        TransactionAccess.GetTransactionInfo(5);
        TransactionAccess.GetIncome(startDate, endDate);
        TransactionAccess.GetExpense(startDate,endDate);
        TransactionAccess.GetFilteredTransactionsList(startDate,endDate);
}


//------------------------------------------------------
class TransactionAccess
{
    private const string connString = "Host=localhost;Username=postgres;Password=1234;Database=ExpenseManagerDB";
    
    public static void GetTransactions()
    {
        using (var connection = new NpgsqlConnection(connString))
        {
            var transactions = connection.Query<TransactionView>(@"SELECT t.transaction_id,t.account_id,a.account_name, a.type,t.note, t.amount, t.date
                                                               FROM account AS a
                                                               INNER JOIN transaction AS t ON a.account_id = t.account_id");
            transactions.Dump();
        }
    }

    public static void GetTransactionInfo(int id)
    {
        using (var connection = new NpgsqlConnection(connString))
        {
            var transactionInfo = connection.Query<TransactionView>(@"SELECT a.account_name, a.type, DATE(t.date), t.amount, t.note, t.transaction_id 
                                                                  FROM transaction AS t 
                                                                  INNER JOIN account AS a ON t.account_id = a.account_id 
                                                                  WHERE t.transaction_id = @id", new { id });
            transactionInfo.Dump();
        }
    }

    public static void MakeTransaction(Transaction transaction, Account account)
    {
        using (var connection = new NpgsqlConnection(connString))
        {
            connection.Execute(@"INSERT INTO transaction(account_id,amount,date,note)
                                         SELECT a.account_id,@amount, @date, @note
                                         FROM account AS a
                                         WHERE a.account_name=@account_name", new { transaction.Amount, transaction.Date, transaction.Note, account.Account_Name });
        }
    }

    public static void CreateAccount(Account account)
    {
        using (var connection = new NpgsqlConnection(connString))
        {
            connection.Execute(@"INSERT INTO account(account_name, type)
                             VALUES(@account_name, @type)", new { account.Account_Name, account.Type });
        }
    }

    public static void GetIncome(DateTime startDate, DateTime endDate)
    {
        using (var connection = new NpgsqlConnection(connString))
        {
            var Income = connection.Query<TransactionView>(@"SELECT SUM(t.amount) as Income 
                                                        FROM transaction AS t 
                                                        INNER JOIN account AS a ON t.account_id = a.account_id 
                                                        WHERE t.date BETWEEN @startDate AND @endDate AND a.type = 'income'", new { startDate, endDate });
            Income.Dump();
        }
    }

    public static void GetExpense(DateTime startDate, DateTime endDate)
    {
        using (var connection = new NpgsqlConnection(connString))
        {
            var Expense = connection.Query<TransactionView>(@"SELECT SUM(t.amount) as Expense 
                                                        FROM transaction AS t 
                                                        INNER JOIN account AS a ON t.account_id = a.account_id 
                                                        WHERE t.date BETWEEN @startDate AND @endDate AND a.type = 'expense'", new { startDate, endDate });
            Expense.Dump();
        }
    }

    public static void GetFilteredTransactionsList(DateTime startDate, DateTime endDate)
    {
        using (var connection = new NpgsqlConnection(connString))
        {
            var filteredTransactions = connection.Query<TransactionView>(@"SELECT a.account_name, a.account_id, a.type, DATE(t.date), t.transaction_id, t.amount, t.note 
                                                                       FROM transaction AS t 
                                                                       INNER JOIN account AS a ON t.account_id = a.account_id 
                                                                       WHERE t.date BETWEEN @startDate AND @endDate
                                                                       ORDER BY t.date", new { startDate, endDate });
            filteredTransactions.Dump();
        }
    }

}


//-----------------------------------------------------------------------------------
public class Transaction
{
    public int Transaction_Id { get; set; }
    public int Account_Id { get; set; }
    public decimal Amount { get; set; }
    public string Note { get; set; }
    public DateTime Date { get; set; }
}

public class Account
{
    public int Account_Id { get; set; }
    public string Account_Name { get; set; }
    public string Type { get; set; }

}

public class TransactionView
{
    public int Transaction_Id { get; set; }
    public int Account_Id { get; set; }
    public decimal Amount { get; set; }
    public string Note { get; set; }
    public DateTime Date { get; set; }
    public string Account_Name { get; set; }
    public string Type { get; set; }
    public int Income { get; set; }
    public int Expense { get; set; }
}

so based on the above you might have understood I am calling every method from the Main() method

but I have two questions here:

1. I was asked to separate the access layer from business object. I Know that from access layer(here it is class Transaction access).I don't understand what business object is here and how to separate it.

2. when I am calling each and every method I am making new connections i.e., var connection = new NpgsqlConnection(connString), but instead I want to use only a single connection for every query, I am excuting. How can I do that.

Please, I am so tired of thinking these two questions. can someone please help me?