This game is quite simple in how it uses the Atari 2600 hardware. Atari Hardware can draw exactly 6 things on a scanline: Playfield, Ball, Sprite 1, Sprite 2, Missile 1, Missile 2. So Adventure made the player be the Ball, the thin walls (seen on two rooms) be the missile, and two of game objects become the two sprites. There is also the torch-light sprite that can take the place of one of the game objects in dark ro…
...in 4KB. Racing the beam - https://en.wikipedia.org/wiki/Racing_the_Beam
The only real time you need to handle things specially is that some room exits are conditional based on whether Game 1 is selected or not.
Because it was so simple to reuse the rooms, there was enough empty space in there for the Easter egg. The "Created by Warren Robinett" sprite alone is a whopping 96 bytes of sprite data.
But you don't need all that much special code to get the Easter egg into the game. You define one additional room that reuses an existing room layout, and add one more sprite to the list of sprites. The sprite is pre-placed in the new room, as defined by the data that initializes all the game objects. The code for the dot mostly uses the existing code for objects, you can pick up and drop objects already, and the dot is no different in that respect. There is a little bit of added code for removing the right side barrier when it's in the correct sprite slot (easy to happen when three objects are in the room), and there's also a little bit of extra code to override the right-side exit for the room.