Where did future Trunks go after the Goku Black Saga? The planet was destroyed by Zeno-sama. So where did they head to with the time machine?
They didn't head back anywhere in the time machine. They went to an entirely different timeline one that Black and Zamasu couldn't affect because it was made after Trunks and Black had already begun their battle.
However because of the concept of duality they would go to a timeline where essential clones of themselves existed. However they didn't have a problem with it.
Whis gave a pretty accurate description in the post-battle episode so I recommend watching it for a fully fleshed out description, the manga currently just reached Black's first Super Saiyan Rose transformation.