As you go through the core story of Final Fantasy 7 Remake, you will eventually reach Wall Market and meet Don Corneo before returning to Sector 7 through the sewers. Your first trip here is mandatory, but once you clear this area and go to the slums of Sector 7, you won’t be able to go back.
However, once you get to Chapter 14, you will open a new set of side quests, one of which requires you to go back to the sewer system. Since there’s no clear way to go back there, that’s what you need to know.
Basically, you need to make some progress on the story in Chapter 14 and go back to Don Corneo’s mansion. That’s where you’ll meet Leslie, who will take you back to the sewers. Talk to him at the mansion, and you can go back to the level.
Once inside, do what you need to do for the side quests, and make sure to complete this task before you leave the area, because you can’t go back until you pass the game.