Cyberpunk 2077's drive-in theater code is something of a joke played on the players by CD Projekt Red. The theater, located in North Oak, is the site of the quest Blistering Love. Although it's possible to find the code in-game, or to circumvent the door entirely, the code is the most straightforward way to access the theater.
Cyberpunk 2077 Drive-In Theater Code: How to Get Into the North Oak Theater
The drive-in theater's door code is 0000. That's right, it's the simplest possible code. Maybe the owner of the theater doesn't take home security very seriously, or maybe CD Projekt Red is making a joke about elaborate codes in other video games. Either way, entering that code will get you access to the theater.
The code can be found in-game by going through the files and messages on the computer in a room in the theater. Players can also get in by using the double jump ability. However you get in, you'll need to head up to the projection room and start the movie to continue the quest.