Virgil worked, as all ancient poets (eg Homer) would have done, by speaking verse out loud while a slave or two transcribed his words.
His style was to come up with perhaps 20 or so lines a day, but then to edit, cut, change those lines relentlessly until only about 3 lines were left at the end of the day.
Some ancient literary critic commenting on this self-editing said that Virgil was like
a she-bear licking her cubs.
Now excuse me. I have some cub-licking to do.