Given the addendum from OP: https://www.quora.com/Were-the-c… (which I’ve added to question details):
The bulk of Ancient Greek drama that has survived has survived as part of the postclassical school curriculum, and has been transmitted through manuscript. Even so, we know that bits of the text that the authors must have written (for the text to make sense) has been left off or garbled. Not a huge amount—a verse here, a couple of verses there; but enough that editors exercise their own ingenuity when reconstructing the complete text, and different editors’ of the dramatic texts will be different.
(Usually, it’s the editors, not the translators doing the conjecturing.)
Outside the manuscript tradition, we have significant chunks of Menander in papyrus, but classical dramatists indeed survive only in small fragments; and for the most part, these aren’t snatches of papyrus, but one or two verses quoted here and there by later authors—usually grammarians.