C# - Creating controls dynamically and accessing them

If I un-comment out those 2 lines(under MenuItem click event), which creates 2 instances of TabPage and RichTextBox, everything works as I want.

When you uncomment those lines, you are adding the same instance of the rich textbox and tab page to the container panel again which is meaningless. Instead add new controls foreach tabpage. (I hope thats the requirement)

Now my first question is why do I have to make new instances of only those 2 types(TabPage, RichTextBox) again but not TabControl?

TabControl is the parent control which has TabPages as child controls. You can have multiple tabs under one TabControl. So you need not create TabControls other than the tbcEditor you have already added. We do not add container controls more than once (unless its the requirement). Do we need more forms? No, just one form which can hold all the child controls right. Similarly just one TabControl which can hold a collection of TabPages. You would need more TabControls only if you want sub-tabs foreach new tab which I guess is not the requirement..

But I can't access rtb since it is declared as local.

This is no big deal. You can do in two ways:

1) Search for your appropriate control by looping. The SelectedTab property gives what you want.

 private void someEvent(object sender, EventArgs e)
 {           
        foreach (Control c in tbcEditor.SelectedTab.Controls)
        {
            if (c is RichTextBox)
            {
                Clipboard.SetDataObject(((RichTextBox)c).SelectedText);
                break; //assuming u have just one main rtb there
            }
        }
 }

2) Tag each rtb to the tabPage when you create it, and then you can get the tag element of the selected tab page to get the rich text box. I would go for this approach.

Edit: (In general pls make the following changes too to your code):

 TabControl tbcEditor = new TabControl();

 private void frmTextEditor_Load(object sender, EventArgs e)
 {
      Controls.Add(tbcEditor);
      tbcEditor.Dock = DockStyle.Fill;
      AddMyControlsOnNewTab();
 }

private void AddMyControlsOnNewTab()
{
    TabPage tbPage = new TabPage();
    RichTextBox rtb = new RichTextBox();
    tbPage.Tag = rtb; //just one extra bit of line.

    tbcEditor.TabPages.Add(tbPage);
    tbPage.Controls.Add(rtb);
    rtb.Dock = DockStyle.Fill;
}

private void newToolStripMenuItem_Click(object sender, EventArgs e)
{
    AddMyControlsOnNewTab();
}

Now, you can call it like this:

 private void someEvent(object sender, EventArgs e)
 {           
        RichTextBox rtb= (RichTextBox)tbcEditor.SelectedTab.Tag;
        Clipboard.SetDataObject(rtb.SelectedText);
        //or even better in just a line,
        //Clipboard.SetDataObject(((RichTextBox)tbcEditor.SelectedTab.Tag).SelectedText);
 }

What you have to consider here is which is the control that you first get and which is the one you do not get. You would get TabPage anyways but not the RichTextBox. So you have to tag RichTextBox to TabPage. You have to cast it since Tag is of type object, so you have to specify which kind of object it is. Finally, this method has the advantage that you need not loop through a list, so its more performant. And that you can have more RichTextBoxes in the TabPage (provided you want to copy text from only one set of RichTextBoxes, one from each TabPage)..