In Genesis, the Ark’s described entrance was a door.
Genesis 6:16 instructs Noah to make a door in the side of the Ark. Later, Genesis 7:16 says that Noah and the animals entered, and that God shut the door behind them. The passage does not describe a ramp, staircase, hatch, or multiple entrances.
The same construction verse mentions an opening or window near the top, but that feature is presented separately from the door. Translators differ slightly over the exact wording and placement of the window, yet the entrance is consistently identified as a door.
Popular artwork often adds gangways, loading platforms, or several openings to make the scene easier to imagine. Those additions are not specified in Genesis. The text gives only a limited architectural description: compartments, three decks, a window or opening, and a door in the side.