how to pass data from a DataTable to a table in MySQL in c#?

From what I see is that you are not executing the insert, in your code:

    foreach (DataRow registro in dt.Rows)
    {
        string especie = (string)registro["planta"];
        int mesventa = (int)registro["mes"];
        int yearventa = (int)registro["year"];
        var cantidad = registro["cantidad"];
        var total = registro["total"];

        string consulta = string.Empty;

        consulta = "INSERT INTO ventas VALUES ('" + especie + "','" + mesventa + "','" + yearventa + "','" + cantidad + "','" + total + "')";


        comand = new MySqlCommand(consulta, new MySqlConnection("Database=vivero; Data source=localhost; User Id=root; Password=; SSL Mode=None"));
        **comand.ExecuteNonQuery();**

    }

Missing command.ExecuteNonQuery(); That's what the insert does to your DB, however you need to give your code more structure. In the button click event you are doing too many things, that is not a good practice. I see that you make many connections to the database and you only do an insert, just as I see that you have the parameters to your DB in your code, that is very bad practice because if you change the database username or password you will have to edit the code and complicate another version.

I leave you an example of the insert:

private void button1_Click_2(object sender, EventArgs e)
{
    // CADENA DE CONEXION A LA BASE DE DATOS
    string cadenaConexion = "Database=dprn3u3; Data source=35.197.23.46; User Id=estudiante; Password=dprn3pass; SSL Mode=None";
    string cadena = "Database=vivero; Data source=localhost; User Id=root; Password=; SSL Mode=None";
    MySqlConnection conexion, comando2,conexion3;
    DataTable dt = new DataTable();
    MySqlDataReader resultados;
    MySqlCommand comand = null;

    try
    {
        // ESTABLECE LA CONEXION A LA BASE DE DATOS
        conexion = new MySqlConnection(cadenaConexion);
        comando2 = new MySqlConnection(cadenaConexion);

        // CONSULTA SQL
        MySqlCommand comandos = new MySqlCommand("SELECT planta, MONTH(fecha) AS mes, YEAR(fecha) AS year, SUM(cantidad) as cantidad, SUM(total) as total FROM ventas GROUP BY year,mes,planta;", conexion);
        MySqlCommand mysql = new MySqlCommand("SELECT COUNT(*) FROM ventas;", comando2);


        // DECIMOS EL TIPO DE CONSULTA 
        comandos.CommandType = CommandType.Text;


        // ABRE LA CONEXION A LA BASE DE DATOS
        conexion.Open();
        comando2.Open();

        // REALIZA LA CONSULTA Y LA ALMACENA EN LA VARIABLE RESULTADO
        resultados = comandos.ExecuteReader();



        // CARGA LOS REGISTROS OBTENIDOS EN LA TABLA Y EN EL STATUSTRIP 
        dt.Load(resultados);


        int contar = Convert.ToInt32(mysql.ExecuteScalar());
        etiquetavalores.Text = contar.ToString() + " " + "registros";

    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }

    database.DataSource = dt;

    //Generamos un StringBuilder que contendra los valores a insertar
    StringBuilder _valoresParaInsertar = new StringBuilder();
    foreach (DataRow registro in dt.Rows)
    {
      if (_valoresParaInsertar.length() > 0)
          _valoresParaInsertar.Append(",")
      //Agregamos valores
        _valoresParaInsertar.Append($"('{(string)registro["planta"]}','{(int)registro["mes"]}','{(int)registro["year"]}','{registro["cantidad"]}','{registro["total"]}')");
    }
    //Hacemos la insercion
    int _registrosInsertados = InsertaDatos(_valoresParaInsertar.ToString());
    Console.WriteLine($"Se han insertado {_registrosInsertados} documentos");
}

The insert function:

private int InsertaDatos(string valoresParaInsertar)
  {
    using (MySqlConnection conexion = MySqlConnection("Database=vivero; Data source=localhost; User Id=root; Password=; SSL Mode=None"))
    {
      conexion.Open();
      //Query de insert
      comand = new MySqlCommand($"INSERT INTO ventas VALUES {valoresParaInsertar};", conexion);
      int _registrosGuardados = comand.ExecuteNonQuery();
      return _registrosGuardados;
    }
  }