The example images on the Tcl tutor website
(
http://www.msen.com/~clif/TclTutorTour.html) [NOTE - you should have
supplied this url) of the Tcl tutor interface appear as if it is itself
Tcl, and that the text boxes are Tk text widgets.
In which case, the documentation of the normal Tk text widget keyboard
bindings would be useful to you:
https://www.tcl.tk/man/tcl8.5/TkCmd/text.htm#M162
Unless Tcl tutor is doing something to replace these bindings, these
are the keys that "do something" within the various views.
The one you want is #18:
Control-/ selects the entire contents of the widget.
Afterwich, you should be able to do one of:
Control-c or key #20:
The F16 key (labelled Copy on many Sun workstations) or Meta-w copies
the selection in the widget to the clipboard, if there is a
selection. This action is carried out by the command tk_textCopy.
To copy the selected text to the clipboard.