According to Genesis, God shut Noah into the Ark after the occupants entered.
Genesis 7:16 says that Noah, his family, and the animals entered as commanded, and then “the LORD shut him in.” The verse presents the closing of the Ark as an action attributed to God, rather than describing Noah or one of his sons fastening the door.
This detail follows the instructions in Genesis 6 and 7, where Noah is told to make the Ark and bring his household and the animals inside. It also marks the transition from preparation to the Flood itself. Genesis does not identify a separate angel as the one who closed the door.
The verse is sometimes paraphrased in retellings, but its wording matters: Noah is the builder and occupant, while the biblical text assigns the closing action to the LORD. The passage does not describe the mechanism of the door or its dimensions.