Door 7 is one of the second set of numbered doors the group encounters. It is located on C Deck inside the large hospital room. It leads to the Operating Room. It is directly to the right of the unmarked door.
After an unsuccessful search for Snake, everyone regrouped and decided to move forward. Lotus proposes to leave one member behind, due to incompatibility of digital roots, which would make it impossible for all 7 of them to pass through any of the doors. Ace volunteered to stay behind, injecting himself with Soporil-Beta so that Junpei and June were forced to leave him behind, regardless of what they thought on the matter. He also did this to prevent anyone from entering Door 3, as there would be no possible combination for all other members to enter doors and enter Door 3, and discovering the mangled corpse in the shower room.
That decided, the group of 6 were left to choose which door they wished to enter, and Junpei made his choice.
With Ace out of the picture, the digital root of the remaining group is:
3+4+5+6+7+8 = 33 = 6
which implied that the only way for them all to go through was if they take Doors 7 and 8 (7+8 = 15 = 6), hence making Door 3 unavailable, unless one was prepared to leave even more people behind.
No valid distribution of the remaining party among those two doors would have allowed for Junpei and June to stay together, so that, due to the further mutual desire of Seven and Lotus not to be together, the player only got to choose between Doors 7 and 8 themselves, with the parties pre-determined, even if technically other valid choices (even satisfying the Seven/Lotus requirement) would exist.
Behind Door 7
Upon entering, the party found a short hallway, with the DEAD placed next to a nearby door. This was the only door in the hallway and it had a plaque above it, indicating that it led into the Operating room.